Unit parser break *_H2O pressure units

[dopplershift]

Not a huge deal, but this breaks compatibility with standard Pint functionality:

from pint import UnitRegistry
ureg = UnitRegistry()
ureg.m_H2O

gives meter_H2O

But with MetPy:

from metpy.units import units
units.m_H2O

gives UndefinedUnitError: 'm_H' is not defined in the unit registry. This is undoubtedly from code trying to use a regex to handle UDunits-style multiplication.