Largest element in a matrix.

::

 mmax(DM x) -> DM
 mmax(SX x) -> SX
 mmax(MX x) -> MX

Extra doc: https://github.com/casadi/casadi/wiki/L_1d9

Doc source: https://github.com/casadi/casadi/blob/develop/casadi/core/generic_matrix.hpp#L997

Implementation: https://github.com/casadi/casadi/blob/develop/casadi/core/generic_matrix.hpp#L997-L999

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