Pull code from `Track` object into helper functions with tests.

[willgraf]

Added several new functions all in utils.py with some basic tests:

• relabel_sequential_lineage
• is_valid_lineage
• get_image_features (this is then used in tracking.py in addition to Track

Pulling these out to improve code readability, make testing easier, and DRY out the codebase.

Additionally, I renamed the test_utils functions to not be public functions (i.e. they are not prefixed with _).