Merge remote branch 'origin/feature/6871_save_proton_charge'
Merge remote branch 'origin/feature/6858_fitfunc_wiki'
Allow UnitFactory to create units from Python. Refs #6882
Export Label unit to Python. Refs #6882

This can then be used as an arbitrary label unit on axes.
Axis::setUnit(name) now returns new unit

This avoids having to retrieve the new unit immediately after setting
it. Refs #6882
Merge branch 'feature/6882_export_unitfactory' into develop