monochromator framework refurbishment: a cleanup of !4529

fix #3286 (closed)

• Integrate Antonia's changes
• rename MonochramatorBase in Monochromator to not write a class for a single xtal * monochromator
• add available_xtals (None=all xtals) to select which xtals are available from an xtals object + a set(xtal) method
• Create an xtal_change object with the available xtals from xtal object
• Trackers must be an option
• Trackers must be reference by mono BUT config outside
• Create a trackers interface in mono with only user needed functionalities
• re write info
• Manage unit for energy and bragg motors in mono (default is keV and deg in xtal object)
• doc
• tests