[BugFix] GMSH interface handling of PhysicalNames-Tag

Changed GMSH reading a bit:
* Element-reading is extracted to an extra method
* Reading is now independed of tag-order
* PhysicalNames tag is now handled (in the sense that the related lines are correctly skipped)

I don't have much test data but everything seems to work fine based on the three examples I tested (incl. the one which wasn't handled correctly before the changes).