Implement continuous_to_discrete().

This function provides a descrete time approximation of a continuous system
assuming a zero order hold.  Unfortunately, neither Eigen nor LAPACK provide
matrix exponential functionality, so I had to implement a naive Taylor serious
matrix exponential.  The implementation is very simple, but may fail for some
matrices, so this function should not be considered production grade.
Computing the matrix exponential is non-trivial and can fail in all sorts of
unexpected ways, even for trivial matrices.  For more information, see
doi://10.1137/S00361445024180.