silence intended UserWarnings in tests

[tv3141]

The testing logs contain hundreds of UserWarnings. The vast majority seems to be:

/home/travis/miniconda/envs/test-environment/lib/python3.6/site-packages/scitools_iris-2.1.0.dev0-py3.6.egg/iris/fileformats/_pyke_rules/compiled_krb/fc_rules_cf_fc.py:2029: 
UserWarning: Gracefully filling 'lat' dimension coordinate masked points

and

/home/travis/miniconda/envs/test-environment/lib/python3.6/site-packages/scitools_iris-2.1.0.dev0-py3.6.egg/iris/coords.py:1210: 
UserWarning: Collapsing a non-contiguous coordinate. Metadata may not be fully descriptive for 'latitude'.

Expected warnings that are intended should be silenced. As an example, Improver has a decorator that does this.

If there are no objections I will have a go at creating a pull request.

[pelson]

I think it is time that we considered silencing these warnings by default in all cases, TBH. Would you be interested in having a go at seeing what that actually looks like in practice? To be clear, I'm not suggesting we remove the warnings, I'm suggesting that they are silenced by default (like the standard DeprecationWarning in python).

[tv3141]

I can have a look at this. Do you prefer disabling UserWarnings only for testing, or in general?