New GmshReader Interface does not provide runtime-flexitbility

The new GmshReader interface introduced in !381 (merged) makes it impossible to pass meaningless parameter combinations. Passing vectors to fill with data always implies reading that data too, and also implies presence of a GridFactory needed to index that data later. However, some people provide interfaces on top of GmshReader that encapsulate the reading in such a way that they only know at runtime whether data will be read (!381 (comment 64973)). Providing such functionality on top of the new GmshReader interface is painful, as it requires to do a switch for all six possible combinations.