Revert the recent changes to Atmosphere.cpp. They were actually in

Revert the recent changes to Atmosphere.cpp.  They were actually in
the wrong place.  The Atmosphere::getStd*() calls are used by the
solver, and thus really need to return values for a "standard"
atmosphere.  Otherwise, an aircraft started up in Moscow will behave
differently than one initialized in Cairo. :)

The place where environmental pressure and temperature get inspected
at runtime is in YASim.cxx.  The changes there, happily, end up being
even smaller than the ones to Atmosphere.  This ends up replacing code
only, and removing some comments.