#1220 CIF to mCRL2 transformer: support most state invariants

• Best to review per commit.
• End-user visible changes:
  • The CIF to mCRL2 transformer now supports state invariants that do not depend on input variables.
• Other changes:
  • InvNoTimeDependentCheck
    • Improved/fixed some local variable names.
    • Renamed to InvNoSpecificRefsCheck. Now also supports disallowing input variable references in invariants.
  • CifChecksTestApp now properly chains exceptions in case a test class can't be instantiated.
  • state_evt_excl_invs.cif test: fixed multiple consecutive empty lines.

Addresses #1220 (closed)