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