Adding back handleLArCustom() to GeoShapeAction.
requested to merge rbianchi/atlasexternals:master-fix-larcustomshape-update-geomodelcore into master
This is needed to fix the Athena master build. When removing the LArCustomShape from GeoModelIO, in fact, all occurrences of LArCustomShape were removed from GeoModelKernel as well; but Athena still uses them, in particular the handleLArCustom() method of the GeoShapeAction class.
For details: