Make required arguments passed through ... explicit

[jamesmbaazam]

Some ... (e.g., in likelihood()) are passed to child functions several levels deep. This makes it very difficult to know which arguments this function is taking and where they are documented.

The nobs_offspring argument is a good example of that. It's required if you passed a distribution that epichains doesn't know but it's not really explicit from the function documentation. And as far as I can tell, you cannot arrive to the help page describing it from links in likelihood().
I am not yet completely sure about the solution for this. Maybe make these arguments explicit rather than implicit in the ellipses?

Originally posted by @Bisaloo in #122 (review)