Improve code related to setting up of coupling variables

[bishtgautam]

• Remove MPPGovEqnSetInternalCouplingVars() and MPPGovEqnSetCouplingVars() as procedures

• Rename MPPGovEqnAddCouplingCondition() to MPPGovEqnAddBoundaryCouplingCondition

• Rename MPPGovEqnSetCouplingVars() to MPPGovEqnSetBoundaryCouplingVars

• Rename MPPGovEqnSetBothCouplingVars() to MPPGovEqnSetCouplingVars()