Refactor code repetition in the time module

[SamuelBrand1]

This looks like it works!

I think there is some unnecessary repetition e.g. in get_observation_indices where I think it would be cleaner to have an int pass into a function rather than if freq == "mmwr_weekly" then that int is 5 etc.

However, maybe lets make that a small refactor issue.

Originally posted by @SamuelBrand1 in #600 (review)

I think for sustainability it would be nice to go back through this and cut out repetition code.