GFS reference pressure

[JoyMonteiro]

GFS uses two reference pressures -- one to set the atmospheric mass, which is taken from sympl constants, and another for calculation of the semi-implicit matrices which is hard-coded at 800 mb (and a reference temperature which is also hard coded at 300 K). These values are defined in semimp_data.f90 line 124.

These values should also be settable externally. This is not a big deal for the upcoming release, but opening an issue so that we don't forget.