feat: (X)EGU support.

[nshehz]

No description provided.

[mhier]

As far as I understand you are introducing new XML tags in the <basename>-DoocsVariableConfig.xml mapping file to specify fixed engineering units to the axes. ChimeraTK already has support for engineering units attached to PVs, and I think we should use that instead. The Y axis unit can be taken from the array PV directly, and the X axis unit can be taken from the incrementSource if present. If instead of an incrementSource we have a fixed increment value in the XML mapping file, we should specify the unit with that value somehow.