Add new check_mods function for generators

[jtraglia]

This function is meant to prevent untested modules in generators. Sometimes we forget to update the all_mods dictionary in a generator and it's difficult to tell; typically a client team will reach out to us.

Given all_mods and the package name (eg epoch_processing), check_mods will point out when there's a missing or unexpected module in all_mods. It does this by comparing all_mods to the modules list for that package.

[hwwhww]

I can see it will save my day. Thank you @jtraglia !