Adding attribute to cmesh while deriving.

[holke]

At the current state it is not possible to add attributes to a cmesh during derivation.
I.e:

t8_cmesh_derive (new_cmesh, committed_cmesh);
t8_cmesh_add_attribute (new_cmesh, ...);

The attributes are not copied over during the commit phase and are returned as NULL when queried.

1. Dont allow this usage, resulting in an error message when adding attributes to a cmesh that is about to be derived.
2. implement the feature to allow for adding attributes.

I implemented a test for the new cmesh vertex IDs that uses this and breaks: https://github.com/DLR-AMR/t8code/blob/feature-cmesh_vertex_connectivity/test/t8_cmesh/t8_gtest_cmesh_vertex_conn_tree_to_vertex.cxx

Edit: There is a standalone test now available (see #972) at: test/t8_cmesh/t8_gtest_cmesh_add_attributes_when_derive.cxx

[holke]

In #972 we implemented an error message when users try to add attributes while deriving.
We also add a (deactivated) test case.

This does not close the issue, since the functionality is still desired.

However, will only be implemented when/if the attribute handling is rewritten completely.