Replace PVG with PDG

[lamont-granquist]

Replaces PVG (indirect method solver) with PDG (direct method with sparse SQP solver).

• This is mostly just feature compatible with PVG, no new features.
• It may fix bugs, particularly around coasting.
• A feature to implement very soon is min coast times--which will fix any lingering "PVG instantly finishes on the pad (because of a negative coast)" bugs.
• At the same time, can support max coast times to stop rockets burning up with overly long coasts.
• Coast-within-stage (in addition to before-stage and after-stage) should also be pretty easy.
• Only major problem right now is that it is slow to converge on the pad.
• It isn't quite as good with finding solutions to badly posed problems (e.g. burning a rocket down to the last 12 kg).
• Successive convexification (SCP) and switching to the alglib sparse SOCP solver should fix the slow solver issue.
• Can be expanded to add (even with SCP):
  • backpressure
  • drag
  • max heat flux
  • pitchover optimization
• The thrust pointing constraint of landing guidance could probably added for ascent from airless planets and not smacking into the terrain.

The last version with PVG is here: https://ksp.sarbian.com/jenkins/job/MechJeb2-Dev/1473/