Support conversion in function parametric::evaluate

[volkm]

Added automatic conversion to the desired return type in parametric::evaluate. This allows to replace the common construct

storm::utility::convertNumber<ConstantType>(storm::utility::parametric::evaluate(function, valuation));

by the simpler version

storm::utility::parametric::evaluate<ConstantType>(function, valuation);

If the types are equal, then of course no conversion is performed.

Drawback is that the include constants.h is now moved to the header file parametric.h but I think this is acceptable.

[tquatmann]

I'm not sure about this, but if we really want to avoid the additional header inclusion of constants.h in parametric.h, we could use explicit template instantiations in the cpp file (for double, cln rationals, and gmp rationals).

Apart from that: LGTM!