confirm that sentence index is sequential across all files within the zipfile

[rlskoeser]

current logic in base input class should do what we want, since we are iterating over all xml files in the zipfile, but let's confirm and add a unit test to verify

[tanhaow]

1. Ran a full volume and confirmed that sentence index is sequential across all files within the zipfile.
2. Added test_get_sentences_sequntial in test_alto_input.py to assert that the collected sent_index values are starting at 0, continuing across all sentences, and matching exactly range(num_sentences).