update use-cases diagram with proposed load (deserialise) XML methods
update use-cases diagram with proposed load (deserialise) XML methods

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