Improve handling of non-gravs

[Bill-Gray]

The code for non-gravitational forces (solar radiation pressure, comet model, with others such as Yarkovsky to be added) is messy. Constrained non-gravs are poorly implemented, and sometimes result in being unable to determine an orbit. This has to be fixed before Yarkovsky or YORP or other models are added. Also, one sometimes wants to have only tangential forces added (i.e., A1=0, and A2 is the only desired parameter).