ForceField: adding const getMState()
ForceField: adding const getMState()

Former-commit-id: a6dd6d5d236bee849ab481336aa1d8e560d8e40e
Anatoscope/sofa,FabienPean/sofa,hdeling/sofa,Anatoscope/sofa,FabienPean/sofa,hdeling/sofa,FabienPean/sofa,hdeling/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,Anatoscope/sofa,FabienPean/sofa,hdeling/sofa,FabienPean/sofa,FabienPean/sofa,hdeling/sofa,FabienPean/sofa,Anatoscope/sofa,Anatoscope/sofa,Anatoscope/sofa,hdeling/sofa,Anatoscope/sofa,Anatoscope/sofa,hdeling/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,hdeling/sofa