6.2.1 RC commit

- Kerbal DRE awareness events
- Chute failure message made more generic
- Moved most on screen messages to ScreenMessage
- Added unsafe chute deployment warning messages
- Moved density / densityExponent to AdjustedDensity() (for alternate
density effect calculation)
- Current alternate density divides density by 10 before doing heat
transfer calculations.
- nonessential heat shield fields made non-persistant. (changes to
shield config now apply to existing shields for direction, reflective,
loss and dissipation)
- Shields now insulate attached parts by setting heatConductivity
- Added assembly version to debug window.
- Added soft heat cap to heat shields. (increased dissipation as it
approaches destruction temperatures)
- Implemented difficulty levels (Easy, Normal, Hard)
- Difficulty can be changed per save game.
- Debug menu changes apply only to current difficulty level.
- Added toolbar button (icons courtesy of lajoswinkler)
- Revamped node saving / loading to accomodate difficulty system.