Change XArray projection warning to a logged message

[vardan10]

Using log.warning instead of warnings.warn

[CLAassistant]

All committers have signed the CLA.

[jrleeman]

Thanks for this PR! Here's what CI is complaining about

/home/travis/build/Unidata/MetPy/metpy/xarray.py:9:1: I100 Import statements are in the wrong order. 'import functools' should be before 'import logging'
/home/travis/build/Unidata/MetPy/metpy/xarray.py:9:1: I202 Additional newline in a group of imports. 'import functools' is identified as Stdlib and 'import logging' is identified as Stdlib.

The import block should look like this to calm it down:

import functools
import logging
import re
import warnings

[jrleeman]

Any objections to going forward with this @dopplershift ?