Backlog of breaking changes

[LTLA]

• Change Right_ to EigenVector_, which enables MockMatrix to be turned into an actual abstract interface.
  • Requires this line to use W_next instead of W.col(j), to avoid an implicit construction.
• Remove defaults on ParallelSparseMatrix.
• Remove divide option in the Scaled wrapper, it's faster and simpler to pass in the pre-divided vector.
• Turn Scaled and Centered into subclasses of the new interface, for optional run-time polymorphism.
• Add option to apply Centered by row, equivalent to Scaled.
• Replace void* initial with the safer std::optional<EigenVector_> initial, given that we need to copy it in anyway.
  • Might as well switch the floats in Options to EigenVector_::Scalar actually probably not a great idea as the scalars might not be floats in the future, e.g., complex values.
  • Also switch extra_work with Eigen::Index, given that it's a dimension-related parameter.