Patch from Julian Foad:

Patch from Julian Foad:

- tidies up the update-time-step handling (making it a simple "dt");
- makes the altimeter get a proper pressure, and the (unused) vacuum
  calculation get a proper RPM (*);
- replaces property name look-ups with static pointers to property nodes.

Notes from DPM:

- the static pointers are a very bad idea, but they're only temporary;
  I plan to make FGSteam into a proper subsystem soon, and then they
  can be member variables
- I fixed the patch to get the current static pressure from the
  /environment/pressure-inhg property, so that the altimeter interacts
  properly with FGEnvironment