Bugfix vtk reader data

[sandro-elsweijer]

Describe your changes here:
Fixes a bug in which cmesh attributes of the vtk reader overwrite already existing attributes with the same key and adds assertions to prevent this behavior.

Introduced a package id for the testsuite.

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[Davknapp]

Please resolve the merge conflict

[Davknapp]

Suggested change

static int t8_testsuite_package_id;

we should not define a global variable here.
googletest provides the possibility to construct entities that can be used over multiple test-suites. We could either do that (more work) or set the package ids in the files where we need them.

[Davknapp]

The package Id should not be defined globally. We could use one per teststuit.

[Davknapp]

Does this close #340 (it uses a different approach I think, but addresses the problem?)