Add decorators to potentially skip tests that require optional dependencies

[spencerkclark]

Currently we have tests which require:

• multiprocess

And if we add tests of the notebook they will also require:

• matplotlib
• runipy or pytest-ipynb

None of these are required dependencies for aospy, so we should only run those tests if those libraries are present on someone's system. xarray has some good examples for how to do this in: tests/init.py.

[spencerahill]

See #166 (comment)

@spencerkclark if you think I'm on the right track there, then I can add the same logic for multiprocess and matplotlib

[spencerkclark]

It would be great if you could take care of multiprocess (I took care of matplotlib already; the notebook test is the only one that requires it).

Simultaneously could you add a new CI environment, called like environment-py27-min.yml (e.g. xarray's) that includes only our minimum requirements? This will ensure that our import skips are operating as desired.

[spencerahill]

Good call, will do.

[spencerahill]

C.f. #168, need to catch pytest-catchlog also for test_automate

[spencerahill]

After #172, multiprocess is no longer needed.