Feature: Make vertex conn optional

[sandro-elsweijer]

Closes #1773

Describe your changes here:
The vertex connectivity currently will be built as soon as a global vertex is added.
This adds the possibility to leave the vertex connectivity completely out if not needed.
The vertex conn will now be built if the user sets the cmesh to do so or if a global vertex is added.
In the future the msh or vtk reader can collect this information from the uncommitted cmesh and then read in the global vertices or not.

Note: most of this are removed trailing white spaces because my vscode deletes them during saving a file.

preparation for #1771

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