simpeg/discretize: Documentation refactor

From PR #134 commits from: @lheagy review from: @dccowan Improvements Organization of base classes move base classes to a base directory (closes #128) Docs use napoleon + numpy-style docs to compile the docs (closes #126) convert existing docstrings to numpy-styled docs separate the API documentation from user documentation (closes #127) Testing travis cleanup (previously it was confusing which version of python was being used. We used the python 3.6 image on travis but then downloaded the latest conda - which is python 3.7): now each test suite is clearly labeled use pytest for testing instead of nose Follow ups content to be developed in the "User Guide" (see #149) create a contributor guide (separate pr) that includes info on how we document classes, methods, functions and class attributes (e.g. https://numpydoc.readthedocs.io/en/latest/format.html#docstring-standard) (see #150) pr on SimPEG to ensure it is up-to-date with the changes in the base-class don't cause upstream problems (see simpeg/simpeg#776) Thanks: @leouieda : for your beautiful repo-setup and docs to follow :)