Reimplemented AST internal node checking functions for better clarity
Reimplemented AST internal node checking functions for better clarity

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