Use defaultdict in mapper's Nerve

Types of changes

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

Description Small enhancement to the internals of Nerve.fit. collections.defaultdict is used instead of a vanilla Python dict as suggested in the Python documentation for such cases (see https://docs.python.org/3/library/collections.html#collections.defaultdict, under "defaultdict Examples").

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.
[ ] My change requires a change to the documentation.
[ ] 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.

giotto-ai / giotto-tda

Use defaultdict in mapper's Nerve #552