Make serial argument a string instead of a function

[jamesmbaazam]

Instead of requiring that, and in the spirit of what I understand to be the underlying issue (offspring and serial calling for different approaches), how about changing serial to take a character string (with corresponding random and distribution functions expected to be present in the environment)? My sense is that this would be simpler.

Originally posted by @sbfnk in #25 (comment)

From the conversation in #25, the general agreement was about the inconsistent requirements for specifying the serial and offspring distributions. We have agreed to make the argument for specifying the serial distribution expect a string instead of making the offspring distribution argument expect a function.