Fix nexus parsing for NOMAD

[domna]

See related issue https://gitlab.mpcdf.mpg.de/nomad-lab/nomad-FAIR/-/merge_requests/1731

• One problem was related to a caching issue, i.e., lru_cache always returns the same object if it has been cached already. If this object is altered, the altered object gets returned for subsequent calls of the lru_cached function. See this stackoverflow question for more information: https://stackoverflow.com/questions/54909357/how-to-get-functools-lru-cache-to-return-new-instances. I simply avoid this now by making a copy after the function call. Maybe we'd need to investigate for other objects of this function call and/or in other places if we have the same problem there
• A second problem originated in the get_nxdl_attr_doc function which added attribute nodes twice. This was due to the fact that all attributes which are part of the inheritance chain were added instead of only the lowest child. Maybe this problem also exists for fields.

I made some remarks on the general code quality in #303

[coveralls]

Pull Request Test Coverage Report for Build 8701886089

Details

• 7 of 10 (70.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 77.874%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pynxtools/nexus/nexus.py	7	10	70.0%

Totals
Change from base Build 8647780955:	0.01%
Covered Lines:	2615
Relevant Lines:	3358

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💛 - Coveralls

[sanbrock]

LGTM

[sanbrock]

LGTM