MNT: Clean up thermo tests.
MNT: Clean up thermo tests.

Refactor away from classes and add docstrings to all test functions.

ahill818/MetPy,ahaberlie/MetPy,dopplershift/MetPy,jrleeman/MetPy,dopplershift/MetPy,Unidata/MetPy,Unidata/MetPy,ShawnMurd/MetPy,jrleeman/MetPy,ahaberlie/MetPy