Precision variant for elementwise exponential: x -> exp(x)-1.

::

 expm1(float x) -> float
 expm1(DM x) -> DM
 expm1(SX x) -> SX
 expm1(MX x) -> MX

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

Doc source: https://github.com/casadi/casadi/blob/develop/casadi/core/generic_expression.hpp#L506

Implementation: https://github.com/casadi/casadi/blob/develop/casadi/core/generic_expression.hpp#L506-L508

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