Revised secondsToImpact computation.

This change accounts for powered descent, and for parachutes and atmospheric drag.
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Updated altitude computation

For time-to-impact computation.  Now only applies min(ASL, true) if the current body has an ocean.
Merge remote-tracking branch 'upstream/master'
Revised altitude ASL / true selector.

It will now only select the smaller of ASL and true altitude if mainBody has an ocean, and ASL is above zero (the latter to account for submarine mods and the like).
Merge pull request #4 from MOARdV/master

Revise secondsToImpact computation