RF: Merge various pairs of cost function classes which have large amounts of shared code

The CostFxnCompoundBiasFieldVarianceScaled and CostFxnCompoundWarpFieldVarianceScaled classes are identical with the exception of a single value used during initialisation. This MR merges the two into a single class called CostFnxnCompoundVarianceScaled, which is templated on the WarpField and BiasField classes.

Similarly, the CostFxnBendingEnergy and CostFxnBendingEnergyBiasfield classes have quite a lot of duplicated code, and differ primarily in their implementation of the CostFxn::cost() function. These two classes have similarly been merged into a single CostFxnBendingEnergy class, which is templated on the WarpFieldBSpline and BiasFieldBSpline classes.