Check correctness of OperatorBase::is_mg

[nfehn]

Related to issue #639, it is not clear whether the member variable is_mg is handled correctly in OperatorBase, see

exadg/include/exadg/operators/operator_base.cpp

Lines 76 to 80 in 5327f3a

	// The default value is is_mg = false and this variable is set to true in case
	// the operator is applied in multigrid algorithm. By convention, the default
	// argument dealii::numbers::invalid_unsigned_int corresponds to the default
	// value is_mg = false
	this->is_mg = (this->level != dealii::numbers::invalid_unsigned_int);

@peterrum @kronbichler Could one of you also check the remaining occurrences of is_mg in OperatorBase? If the implementation is correct, we should nevertheless think about a better name, because it seems that is_mg does not necessarily describe whether an object of type OperatorBase is used as a PDE level operator in multigrid or not.

[nfehn]

suggestion for renaming: is_mg -> matrix_free_operates_on_coarser_tria_level

[peterrum]

suggestion for renaming: is_mg -> matrix_free_operates_on_coarser_tria_level

Sounds reasonable.

There are other places where the term is_mg is used. I think similar to #640, we could rename these.