Fix most of cmesh_copy

[lukasdreyer]

Describe your changes here:
Fixes most of the problems that made the cmesh_copy_test fail.

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[Davknapp]

you write, that most of cmesh_copy is fixed by this, what is missing?

[lukasdreyer]

you write, that most of cmesh_copy is fixed by this, what is missing?

There was still a problem with partitioned cmeshes, which currently disjoint_bricks is the only one in our testsuite. I investigated further and found wrong usage of sc_shmem_memcpy. So now, all of cmesh copy is fixed.