Resolve "nexus writer as a separate package"

Closes #4065 (closed)

The purpose of the MR is to create a separate blisswriter package with minimal refactoring.

Merge these first: !5875 (merged), !5872 (merged)

Then rebase this one.

pypi release

v1.0.0b0 of blisswriter has been released (this MR is currently on v1.0.0b1):

https://pypi.org/project/blisswriter/

https://test.pypi.org/project/blisswriter

pip install blisswriter[test]
pytest --pyargs blisswriter.tests

Further releases will be handled like blissdata. CI has a deploy step, is this the official way?

conda release

The CI conda step fails. Not sure how conda packages of blissdata are released but it should be similar for blisswriter. I guess a branch in https://gitlab.esrf.fr/bliss/conda-recipes. Not sure how this relates to the CI conda step of the bliss repo.