Rewritten sizes_to_indices to use np.cumsum.

[MarkCBell]

[mbannick]

@MarkCBell thanks for your patience in reviewing your PR. We've just deployed a large refactor to this code base, and it is no longer being used for COVID-19 forecasting. If you would still like to contribute, please resolve the merge conflict, and add a test to ensure that your updated function returns the same result as our current function. Thank you!