Compile Error Melodic due to tf change

[moriarty]

A change in tf.h here: ros/geometry#163
causes the following compile error on Ubuntu 18.04 and Melodic:

Errors     << amcl:make /home/amoriarty/melodic/logs/amcl/build.make.000.log                                                                                                                               
/home/amoriarty/melodic/src/navigation/amcl/src/amcl_node.cpp: In member function ‘tf2_ros::Buffer& AmclNode::TransformListenerWrapper::getBuffer()’:
/home/amoriarty/melodic/src/navigation/amcl/src/amcl_node.cpp:130:51: error: ‘tf2_buffer_’ was not declared in this scope
       inline tf2_ros::Buffer &getBuffer() {return tf2_buffer_;}
                                                   ^~~~~~~~~~~
/home/amoriarty/melodic/src/navigation/amcl/src/amcl_node.cpp:130:51: note: suggested alternative: ‘tf2_buffer_ptr_’
       inline tf2_ros::Buffer &getBuffer() {return tf2_buffer_;}
                                                   ^~~~~~~~~~~
                                                   tf2_buffer_ptr_
make[2]: *** [CMakeFiles/amcl.dir/src/amcl_node.cpp.o] Error 1
make[1]: *** [CMakeFiles/amcl.dir/all] Error 2
make: *** [all] Error 2

The simple gcc suggestion to replace tf2_buffer_ with tf2_buffer_ptr_ doesn't fix the problem, just gets the next error msg:

Errors     << amcl:make /home/amoriarty/melodic/logs/amcl/build.make.001.log                                                                                                                               
/home/amoriarty/melodic/src/navigation/amcl/src/amcl_node.cpp: In member function ‘tf2_ros::Buffer& AmclNode::TransformListenerWrapper::getBuffer()’:
/home/amoriarty/melodic/src/navigation/amcl/src/amcl_node.cpp:130:51: error: invalid initialization of reference of type ‘tf2_ros::Buffer&’ from expression of type ‘std::shared_ptr<tf2_ros::Buffer>’
       inline tf2_ros::Buffer &getBuffer() {return tf2_buffer_ptr_;}
                                                   ^~~~~~~~~~~~~~~

I was using the lunar branch earlier as it was the most up to date and was working with melodic, but came back from holidays and it fails to compile.

I just brought this up with @aaronhoy, I think to open a pull request to fix this issue requires a melodic-devel branch