[discretefunction.default] add make_...discrete_function()

These can be used quickly obtain discrete functions instead of typing out
all the template arguments the compiler knows anyway:

* `auto const_func = GDT::make_const_discrete_function(space, vec);`
* `auto       func = GDT::make_discrete_function(space, vec);`
* `auto       func = GDT::make_discrete_function< VectorType >(space);`