of MAPHIS should automatically register them (e.g. whether `LabelEditor` should register a `Tool` on its own)

Brush: fix crashing when painting outside constraint-allowed area (Closes #238)

Closes #238

See merge request xmraz3/maphis!5

# Conflicts:
#	maphis/__init__.py

- ignore labels from a label image that are not present in the label hierarchy
- simplified the routine that computes the hierarchy between labels

Merge the final version into main

See merge request xmraz3/arthropod_describer_py!4