Move type-dependent code in xField out of the core of the class
Type-dependent code in xField:
static void getVal (iterFct it, iterFct ite, iterVal first, iterVal last, int size_Fct_Val, const xGeomElem* geo_appro, const xGeomElem* geo_integ, std::complex<double>& v);
and getGrad are located in the core of the class, making it cumbersome to extend to new user-types. We should try to see if we can implement this logic in the user's code.