allow for variable units to be defined without name
allow for variable units to be defined without name

hsorby/libcellml,libcellml/buildbot-testing,MichaelClerx/libcellml,cellml/libcellml,cellml/libcellml,nickerso/libcellml,nickerso/libcellml,cellml/libcellml,nickerso/libcellml,hsorby/libcellml,hsorby/libcellml,libcellml/buildbot-testing,MichaelClerx/libcellml,cellml/libcellml,hsorby/libcellml,nickerso/libcellml,MichaelClerx/libcellml