Add numpy dependency to package.xml.

[nuclearsandwich]

The build is currently failing due to the numpy import in the template
http://build.ros2.orgjob/Dbin_uB64__rosidl_generator_py__ubuntu_bionic_amd64__binary/1/console

06:05:10 [ 89%] Generating Python code for ROS interfaces
06:05:10 cd /tmp/binarydeb/ros-dashing-rosidl-generator-py-0.7.0/obj-x86_64-linux-gnu/rosidl_generator_py_custom__py && /usr/bin/python3 /tmp/binarydeb/ros-dashing-rosidl-generator-py-0.7.0/bin/rosidl_generator_py --generator-arguments-file /tmp/binarydeb/ros-dashing-rosidl-generator-py-0.7.0/obj-x86_64-linux-gnu/rosidl_generator_py__arguments.json --typesupport-impls "rosidl_typesupport_c;rosidl_typesupport_fastrtps_c;rosidl_typesupport_opensplice_c"
06:05:10 ModuleNotFoundError in template '/tmp/binarydeb/ros-dashing-rosidl-generator-py-0.7.0/resource/_msg.py.em': No module named 'numpy'
06:05:10 ModuleNotFoundError when expanding '_idl.py.em' into '/tmp/binarydeb/ros-dashing-rosidl-generator-py-0.7.0/obj-x86_64-linux-gnu/rosidl_generator_py/rosidl_generator_py/msg/_constants.py': No module named 'numpy'
06:05:10 Error processing idl file: /tmp/binarydeb/ros-dashing-rosidl-generator-py-0.7.0/obj-x86_64-linux-gnu/rosidl_adapter/rosidl_generator_py/msg/Constants.idl
06:05:10 Traceback (most recent call last):
06:05:10   File "/tmp/binarydeb/ros-dashing-rosidl-generator-py-0.7.0/bin/rosidl_generator_py", line 40, in <module>
06:05:10     sys.exit(main())
06:05:10   File "/tmp/binarydeb/ros-dashing-rosidl-generator-py-0.7.0/bin/rosidl_generator_py", line 36, in main
06:05:10     return generate_py(args.generator_arguments_file, args.typesupport_impls.split(';'))
06:05:10   File "/tmp/binarydeb/ros-dashing-rosidl-generator-py-0.7.0/rosidl_generator_py/generate_py_impl.py", line 53, in generate_py
06:05:10     generate_files(generator_arguments_file, mapping)
06:05:10   File "/opt/ros/dashing/lib/python3.6/site-packages/rosidl_cmake/__init__.py", line 121, in generate_files
06:05:10     raise(e)
06:05:10   File "/opt/ros/dashing/lib/python3.6/site-packages/rosidl_cmake/__init__.py", line 116, in generate_files
06:05:10     template_basepath=template_basepath)
06:05:10   File "/opt/ros/dashing/lib/python3.6/site-packages/rosidl_cmake/__init__.py", line 175, in expand_template
06:05:10     interpreter.string(template_content, template_path, locals=data)
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 2391, in string
06:05:10     self.safe(scanner, True, locals)
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 2401, in safe
06:05:10     self.parse(scanner, locals)
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 2421, in parse
06:05:10     token.run(self, locals)
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 1547, in run
06:05:10     self.subrun(info[0][1], interpreter, locals)
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 1631, in subrun
06:05:10     token.run(interpreter, locals)
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 1425, in run
06:05:10     interpreter.execute(self.code, locals)
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 2595, in execute
06:05:10     _exec(statements, self.globals, locals)
06:05:10   File "<string>", line 5, in <module>
06:05:10   File "/opt/ros/dashing/lib/python3.6/site-packages/rosidl_cmake/__init__.py", line 221, in _expand_template
06:05:10     interpreter.string(content, str(template_path), kwargs)
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 2391, in string
06:05:10     self.safe(scanner, True, locals)
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 2401, in safe
06:05:10     self.parse(scanner, locals)
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 2421, in parse
06:05:10     token.run(self, locals)
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 1425, in run
06:05:10     interpreter.execute(self.code, locals)
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 2595, in execute
06:05:10     _exec(statements, self.globals, locals)
06:05:10   File "<string>", line 3, in <module>
06:05:10 ModuleNotFoundError: No module named 'numpy'
06:05:10 Exception ignored in: <bound method Interpreter.__del__ of <empy pseudomodule/interpreter at 0x7f83fd338ba8>>
06:05:10 Traceback (most recent call last):
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 2094, in __del__
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 2157, in shutdown
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 2646, in finalize
06:05:10   File "/usr/lib/python3/dist-packages/em.py", line 2200, in push
06:05:10 AttributeError: '_io.TextIOWrapper' object has no attribute 'push'

I'm not sure of the proper scope for this dependency so for this PR I've
opted to make it a blanket dependency in order to proceed with the
release absent feedback.

[nuclearsandwich]

The PR job will fail because test_msgs is not yet released and built. rosidl_runtime_py is currently ignored at release due to thsi dependency cycle.

[nuclearsandwich]

I'm going to merge this as is in order to allow binary builds to continue. Please feel free to comment or give opinion on more specific dependency sets that this belongs to.

[dirk-thomas]

Please feel free to comment or give opinion on more specific dependency sets that this belongs to.

numpy is needed when building the tests of this package as well as when downstream packages are using it. Therefore I think an exec_depend should be sufficient.