Update units.h
Update units.h

MichaelClerx/libcellml,hsorby/libcellml,nickerso/libcellml,cellml/libcellml,nickerso/libcellml,cellml/libcellml,cellml/libcellml,MichaelClerx/libcellml,hsorby/libcellml,nickerso/libcellml,MichaelClerx/libcellml,nickerso/libcellml,hsorby/libcellml,cellml/libcellml,hsorby/libcellml