Create run_kubernetes function

[mcgibbon]

We need to be able to run the model on kubernetes. This PR creates a python function run_kubernetes for doing so. It has not yet been added to the bash fv3run command.

Fixes #33.

Other API changes:

• DependencyError is removed, instead a DelayedImportError object is used to delay the ImportError until the code attempts to access a member of the imported module.

Non-API/internal changes:

• pytest is now used to run tests
• make coverage now uses pytest
• python 3 versions of python binaries are called explicitly (e.g. python3 and pip3)
• internally the package now uses gcsfs instead of gsutil for copy operations. fsspec, which has the same API as gscfs, is now used for local copy operations other than linking
• added an example for creating a run directory using an external path, which I used to manually test remote copy operations
• asset list generation for remote and local directories has been unified into one function
• tree walking responsibility and asset creation responsibility of asset list generation has been separated into two functions
• distinction is made between "get" and "copy" in file operation helper function names
• native fv3run functionality uses a named logger instead of the root logger
• removed unused copy/link methods in _datastore.py
• fixed some code-formatting in sphinx docs

[mcgibbon]

Ah sorry fv3config-python is a typo, it should be fv3gfs-python.