Changes

Make PVG converge under more extreme circumstances

By falling back to numerical integration when initial analytical
integration fails, we can burn more of the rocket and hit more
extreme solutions (closer to burning the rocket to the last proton).

This will give users better feedback when their target conditions
exceed their rocket capabilities.

Signed-off-by: Lamont Granquist <lamont@scriptkiddie.org>
 (commit: 9f4d60a)