feat(slicing): enforce filament flow-type restrictions in auto grouping

Filament grouping already consumed per-filament forbidden nozzle volume
types, but every call site passed an empty map, so a variant-restricted
filament (e.g. one limited to "Direct Drive TPU High Flow") could be
auto-grouped onto an incompatible nozzle flow type on multi-variant
printers.

- add convert_to_nvt_type() to parse extruder variant strings
- add Print::get_filament_unprintable_flow(): forbidden volume types =
  printer extruder variants minus the filament's declared variants;
  filaments declaring no variants stay unrestricted
- feed the map into grouping at the by-object path (Print.cpp) and all
  six mapping/planning sites in reorder_extruders_for_minimum_flush_volume
- unit-test the string parser

Non-restricted configurations produce an empty map, so existing
printers' grouping and g-code are unchanged.

tests/libslic3r/CMakeLists.txt

@@ -21,6 +21,7 @@ add_executable(${_TEST_NAME}_tests
     test_polygon.cpp
     test_mutable_polygon.cpp
     test_mutable_priority_queue.cpp
+    test_nozzle_volume_type.cpp
     test_stl.cpp
     test_meshboolean.cpp
     test_marchingsquares.cpp