This removes all remaining uses of Eigen::DynamicSparseMatrix, which has been deprecated in Eigen for a while.

• removes unreachable code with comment summary of performance comparison in cat.cpp
• removes commented code in diag.cpp
• replaces Eigen::DynamicSparseMatrix with setFromTriplets in diag.cpp
• adds a unit test for igl::diag
• previous slice_into was buggy for sparse inputs ( 🤯 ): adds a unit test and fixes sparse overload (replacing Eigen::DynamicSparseMatrix with use of SparseVector and innerIterators)