Improve parsing of base-clauses in S_define

A base-clause is the stuff in an inheritance list after the colon in a
class declaration.
https://en.cppreference.com/w/cpp/language/derived_class

Closes #723
Closes #733

test/SIM_parse_s_define/models/Classes.cpp

@@ -0,0 +1 @@
+void foo() {};