changed
- `segments` property of a `Sequence` is a tuple, not a list, so that class is immutable + hashable
fixed
- `__hash__` implementation for `Sequence` class
+ convert attributes that are `numpy.ndarray`s into tuples before hashing
- tests for `Sequence`
+ no longer assert that calling `__hash__` raises `NotImplementedError`
+ test that `segments` attribute is a `tuple` not a `list`