Change default value of nan_fill_value in PersistenceEntropy to -1

[ulupo]

Reference issues/PRs

450

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)

Description Favours a default value of nan_fill_value which is guaranteed not to break a pipeline in downstream tasks. -1. is negative and hence cannot be an entropy. None is still available to the user, at their own risk.

Checklist

• [x] I have read the guidelines for contributing.
• [x] My code follows the code style of this project. I used flake8 to check my Python changes.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed. I used pytest to check this on Python tests.