season filter for interval_log_likelihood

[slager]

This will likely be a common enough subset that would be nice to have a function argument for {prebreeding, postbreeding, all (default = all}

interval_log_likelihood() and route_migration() could potentially use a common helper function to parse the season input string and extract the relevant info from the birdflow object.

[ethanplunkett]

I'm close to pushing an update with a helper function to look up time sequence that should cover most use cases including this one.

[ethanplunkett]

The time commit addressed part of this but didn't add the functionality to the interval_log_likelihood(). Reopening.

[slager]

this can probably stay in the issues but can be de-prioritized for now since I'm currently spending more time using other metrics than with looking at single-season log likelihoods.