Corrected error in IBSolver.h.  advanceSubstep(...) must be a virtual method as the SFD derived class overwrites it.  This was accidentally changed in the last revision, but is now corrected.
Corrected error in IBSolver.h.  advanceSubstep(...) must be a virtual method as the SFD derived class overwrites it.  This was accidentally changed in the last revision, but is now corrected.

git-svn-id: 0df3d21deda237bdb68c105db3677c34a480bda5@302 f381375e-2823-4fb2-bad6-22fa55754e8c

cwrowley/ibpm,cwrowley/ibpm,cwrowley/ibpm