Added SRP.SRPModel to user interface spec. Updated ForceModel documentation.
Documentation updates ready for review. See:
DSC - please review documentation updates as noted above.
John M noted a potential point of confusion in how the Pressure value in the SPAD SPO-file is handled. I recommend changing GMAT to completely ignore the Pressure value in the SPO-file, with appropriate associated documentation updates.
Coded as part of Master branch.