Get rid of Python user warnings

[forman]

Expected behavior

When using the CLI, Python user warnings should not be printed by default. We could add an option to enables them.

Actual behavior

We get user warnings from various 3rd-party packages when using CLI. For example:

(cate-env) $ cate-webapi start
/Users/forman/miniconda3/envs/cate-env/lib/python3.6/site-packages/matplotlib/__init__.py:1405: UserWarning: 
This call to matplotlib.use() has no effect because the backend has already 
been chosen; matplotlib.use() must be called *before* pylab, matplotlib.pyplot,
or matplotlib.backends is imported for the first time.

  warnings.warn(_use_error_msg)
started cate-webapi, listening on 127.0.0.1:49433

Specifications

Cate 0.9.0.dev7