Analyser: allow for multi-level units checks (needed for piecewise statements).
Analyser: allow for multi-level units checks (needed for piecewise statements).

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