MNT: New `CostFxnSSDWarpFieldUtils` module containing code shared by the `CostFxnSSDWarpField` cost function classes.

This MR is based on !7 (merged), which should be merged before this is merged.

Merges the CostFxnSSDWarpFieldSymmetricMaskedExcluded and CostFxnSSDWarpFieldSymmetricMaskedExcludedDiagHess classes into a single class, templated on a boolean parameter which dictates how the hessian is calculated. Most of the logic has been converted into standalone functions, located in a new module called CostFxnSSDWarpFieldSymmetricMaskedExcludedUtils.