MNT: Merge `CostFxnTensor*` cost functions into a single class, templated on `CostFxn::HessType`.

Paul McCarthyrequested to merge
mnt/share-costfxntensor-code into master

Common logic has been moved into standalone functions within a new CostFxnTensorL2WarpFieldSymmetricMaskedExcludedUtils module.

Also moved the create_empty_jtj function (defined in several locations) into the CostFxnSplineUtils module.

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