Parachute landing

This is a second attempt at getting the parachute landing stuff right. I have fixed a problem with the drag model, resolved some issues thrown up by dodgy terrain data, separated out the concepts of the decelerationASL and landingASL, fixed the problems with the variable delta-t in the simulation, and various other improvements / fixes.
There are still things that need to be solved - in particular the decent speed policy that is used on Duna needs to be looked at, and the kill horizontal speed step needs to be much more aggressive in killing horizontal speed, both of which may be issues that have been made more apparent by separating out the concepts of landingASL and DecelerationASL. It also might be a good idea to base the decelerationASL off the targetASL (which is constant) rather than the predictedlandingASL which can potentially jump around. We could also consider manually fully opening parachutes by hacking the full deployment height, in order to remove uncertainties of them deploying uneven ground. It would also be a good idea to relook at the policy for abandoning on targeting the parachutes.