RichardsRelPermPowerGas added, and tests added.  This should close #2635
Merge pull request #2648 from WilkAndy/feature_2635

RichardsRelPermPowerGas added, and tests added.  This should close #2635
Merge commit 'fcb149e3455fb68005173e8a7eb1ddb3e88a4a11'