Convert a lower triangular matrix to a symmetric one.

::

 tril2symm(DM a) -> DM
 tril2symm(SX a) -> SX
 tril2symm(MX a) -> MX

Extra doc: https://github.com/casadi/casadi/wiki/L_1c2

Doc source: https://github.com/casadi/casadi/blob/develop/casadi/core/generic_matrix.hpp#L519

Implementation: https://github.com/casadi/casadi/blob/develop/casadi/core/generic_matrix.hpp#L519-L519

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