Bugfix/double vertex insertion

When inserting two element that share a new vertex this vertex was inserted twice resulting in a disconnected grid. The check if a vertex is already inserted was already in place, however the map had the wrong scope and the pointer written in the map was a pointer to the grid entity not the entity in the to-be-inserted queue.

mentioned in merge request !15 (merged)

mentioned in commit 87c521ad

Status changed to merged

mentioned in commit a33c11bc