Added an Eigen::SparseMatrix formatter.
Also modified the Eigen::Matrix formatter to support Eigen::MatrixXd.
Eigen::MatrixXd sets both dimension template arguments to -1, so they
can't be used for iteration. rows() and cols() are now used instead.
rows() and cols() are constexpr for statically sized matrices, so
there's no performance loss there.
* Replace Matrix<> with Vector<> where vectors are explicitly intended.
I found these via `rg "Eigen::Matrix<double, \w+, 1>"`.
* Pass all Eigen matrices by const reference. I found these via `rg
"\(Eigen"` on main (the initializer list constructors make more false
positives).
* Replace MakeMatrix() and operator<< usage with initializer list
constructors. I found these via `rg MakeMatrix` and `rg "<<"`
respectively.
* Deprecate MakeMatrix()
