Update signature and documentation of Representation::applyCorrection

- Modified name form Representation::applyDofCorrection.  It wasn't
clear whether the "degrees" were being updated, or whether the
associated position and velocity were being updated.
- Added more deatiled documentation.
- Changed name of 'block' parameter to 'deltaVelocity'
Minor fix, modify includes in Fem3DRepresentation.cpp
Minor fix, clarify comment for XXXRepresentation::deactivateAndReset