nexus writer as a separate package
The aim is this:
pip install bliss-nxwriter
This requires decoupling: nothing from bliss
should be imported and blissdata
cannot directly import things from the writer package.
Integration tests from Bliss scans will stay in Bliss.
Add tests and demo example similar to https://bliss.gitlab-pages.esrf.fr/bliss/master/blissdata/getting_started.html#scan-creation-and-streaming.