Update <epidist> utility functions

[joshwlambert]

This PR addresses comments from the full package review (PR #151).

The main changes are:

• Removing possible_epidist_params() (and it's associated documentation and tests)
• Adding information from possible_epidist_params() to the documentation of epidist()
• Removing prob_dist_params argument from calc_dist_params() as it was not needed
• Removing input checking from calc_dist_params() as it is already verified in the higher level functions exposed to the user (i.e. exported functions)

Small, non-functional, updates have been made to several other <epidist> utility functions.