Initialise index of UnitReference to zero.
Initialise index of UnitReference to zero.

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