scimag3d: allow graphing of helium level block

Rob Dalgliesh is currently unable to graph this as the PV $(P)A:HE_LEVEL is linked to a string indicator on the LabVIEW (bottom right of VI, Helium Level meter, level A) and this indicator shows both value and "mm" units. We need this information available as an EPICS analogue output record, changing the existing PV is probably OK, or a new PV could be created. It might be the numeric value is available as a separate indicator elsewhere on the LabVIEW, otherwise the existing string will need splitting within EPICS to remove the "mm"