[discretefunction.default] update default name (remove the 'dune.' prefix)
[discretefunction.default] allow move of ConstDiscreteFunction
[discretefunction.default] clearly doc the need for manual ctors
[discretefunction.default] add ctor which moves in a vector
[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);`
[operators.interfaces] fix apply2 implementations
[operators.prolongations] add prolong()

These can be used to quickly prolong functions without the need to type
all the template arguments the compiler knows anyway:

* `GDT::Operators::prolong(target_grid_view, source, target);`
* `GDT::Operators::prolong(source, target);`
[operators.elliptic-swipdg] whitespace
[operators.elliptic-swipdg] add todo, refs #32
[operators.elliptic-cg] use storage provider
[localevaluation.swipdg] update float_cmp usage