Add motor info to QNexuswidget

[mspito]

Needs #1087 first

[vasole]

Please rebase your work on current master that includes #1087

[vasole]

Some comments:

PyMca is used at other facilities than the ESRF and former ESRF files did not look like current ones.
I find nxpositioners = nxentry.get("instrument/positioners_start", None) very ESRF-centric and restrictive.

I would aim at using something similar to the function getPositionersGroup(h5file, path) in PyMca5.PyMcaCore.NexusTools.py That would make the use of the new feature not restricted to ESRF. For instance, A getStartingPositionersGroup in PyMca5.PyMcaCore.NexusTools.py seems to me more appropriate. It could use your hard-coded specific call and default to getPositionersGroup if nothing found. That would imply that if one positioner is an array of positions, the first position is used as starting point to allow the use of get_motor_positions.

[mspito]

I tried to address your suggestions. Is there anything else to be done?

[woutdenolf]

Suggested change

	It assumes they are either in:
	- NXentry/NXinstrument/positioners_start or
	This method assumes the positioner group is NXentry/NXinstrument/positioners_start.