Misc bugfixes and missing functionality in Xobjects

[szymonlopaciuk]

Description

• Implement _to_dict() on multi-dimensional arrays
• Correctly serialise and deserialise UnionRef fields in structs (may lead to copies though)
• Allow passing None to xo.Arg(..., pointer=True) arguments: these will evaluate to NULL
• Add ThisClass support to Struct kernel arguments

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

Optional:

• The code I wrote follows good style practices (see PEP 8 and PEP 20).
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts