Add function in interfaces to get the solver's method

[cvanaret]

Added a function uno_get_solver_method() in C/Fortran/Python interfaces for the user to check which method is used by the solver, based on the strategy combination. This is especially useful because some special cases in the ingredient instantiation exist (e.g., if unconstrained, Uno automatically picks merit function + no relaxation).

Fixes https://github.com/cvanaret/Uno/issues/421