Allow loading of plugins via entry_points

[domna]

The reader can basically refactored out as is, we just have to add a pyproject.toml with the following entrypoint specification:

[project.entry-points."pynxtools.reader"]
test = "test_reader:MyReaderClass"

where test is the name of the reader referred by --reader and MyReaderClass is the reader class in the file.

A working example can be found in https://github.com/FAIRmat-NFDI/pynxtools-stm for the sts/stm reader.

[coveralls]

Pull Request Test Coverage Report for Build 7021204778

• 8 of 13 (61.54%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.6%) to 51.029%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pynxtools/dataconverter/convert.py	8	13	61.54%

Files with Coverage Reduction	New Missed Lines	%
pynxtools/dataconverter/helpers.py	1	96.28%
pynxtools/nexus/nxdl_utils.py	1	75.16%

Totals
Change from base Build 7017248188:	-0.6%
Covered Lines:	5828
Relevant Lines:	11421

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💛 - Coveralls

[sherjeelshabih]

This works really nicely. I have tested the stm reader and it gets loaded correctly.

As we discussed, the next thing would be to have a testing strategy.

Also, we will need a list of reader plugins in this repo to help users find them.

[domna]

We could build a cookiecutter template to get people started with developing their own readers