Handle Column groups

[kMutagene]

Columns and their term source references and/or associated other columns (such as units) have to be grouped or at least created as groups when inserting them.

Current plan:

• Create Term Source REF and Term Accession Number Columns for every annotation column
• Make these columns Hidden and read-only

[Freymaurer]

i just noticed this issue and this was implemented with PR #34, but the additonal columns are not read-only yet. Will add this with the next PR.

EDIT: I looked into it and read-only will also prevent office.js to write into protected cells. So we would have to unlock the cells we target adn them lock them afterwards again. This would create a large overhead for all office interop functions and i don't know if we gain enough to justify this.

[kMutagene]

Hidden should be enough