numpy_helper_functions.py: Fix functions for finding constant regions in arrays

[andthum]

numpy_helper_functions.py: Fix functions for finding constant regions in arrays

Type of change

• Change of core package.
• Change of scripts.

• Bug fix.
• New feature.
• Code refactoring.
• Dependency update.
• Documentation update.
• Maintenance.
• Other: Description.

• Non-breaking (backward-compatible) change.
• Breaking (non-backward-compatible) change.

Proposed changes

• Fix the functions get_const_seqs, find_const_seq_n and find_const_seq_long that were giving wrong results for completely constant arrays.

PR checklist

• I followed the guidelines in the Developer's guide.
• New/changed code is properly tested.
• New/changed code is properly documented.
• The CI workflow is passing.