Add some more quanities

[samueljackson92]

I would like to add the following quantities to the dataset for MAST and MAST-U:

MAST Name	MAST-U Name	Description
ESM_p_phi	ESM/POWER/POHM	ohmic power
/xdc/z/s/zip	/xzc/Zcon/zip	z position * ip
/xdc/z/t/zref	XDC/ZMANAGE/T/ZREF	z reference

Will need to track down if there is a sensible IMAS mapping