Consistent definition of distributions as function input

[joshwlambert]

The sim_*() functions accept an anonymous function as input for delay distributions, but the definition of the contact distribution and Ct distribution are defined by a character string and a named vector for the distribution parameters.

There doesn't seem any need to have this inconsistency in the package and they both defining distributions.

The create_config() function should be updated to create anonymous functions by default, matching the design of the sim_*() functions; and the internal simulation code should be updated to use anonymous functions instead.