Finished off the atmosphere model (Troposphere and lower

Finished off the atmosphere model (Troposphere and lower
Stratosphere).  The atmospheric properties are as follow:

/environment/temperature-sea-level-degc
/environment/temperature-degc
/environment/pressure-sea-level-inhg
/environment/pressure-inhg
/environment/density-sea-level-slugft3
/environment/density-slugft3

Setting either the sea-level or altitude value automatically sets the
other value appropriate, except for temperature at altitude above the
Troposphere (where there's no reliable way to back-calculate it).  The
atmosphere model appears in the atmosphere_data array in
environment.cxx, and can easily be extended into the upper
stratosphere and beyond.

These are not yet tied into the FDMs or steam module.