A number of minor changes to ExponentialSpringForce.cpp
A number of minor changes to ExponentialSpringForce.cpp

- Found the proper way to access Ground in realizeSubsystemDynamicsImpl(). Added the header file "simbody/internal/MobilizedBody_Ground.h", and now casting Ground to a MobilizedBody& is not necessary.  This change also allowed removing "ground" as a member variable of ExponentialSpringForceImpl.

- Added the SimTK_STAGECHECK_GE_ALWAYS() macro to the data cache accessor methods.

- Corrected a few typos and nits.

simbody/simbody,simbody/simbody