Continued FGViewer cleanups. FGViewer is now a base class so that we can

Continued FGViewer cleanups.  FGViewer is now a base class so that we can
derive specific viewer classes from it.  Here's what I currently have in mind:

FGViewer
  |
  |-> FGViewerPRH     (current system with orientation specified in
  |                    LaRCsim Euler angle convention)
  |
  |-> FGViewerLookAt  Feed in a position, view direction, and up vector
  |
  |-> FGViewerHPR     (similar to PRH, but using ssg hpr euler angle
  |                    convention)
  |-> others?