support polydispersity for integer parameters (e.g., pearl necklace)

[pkienzle]

Models such as pearl necklace would like to be able to have polydispersity on the number of pearls and fit the number of pearls.

Need to be able to tag a parameter as an integer parameter, and use this to select the discrete version of the distribution. The model will still take the integer parameter as a floating point number to keep the code simple.

The current version sort of works as long as the model converts floats to integers internally. With enough points in the distribution, each integer will be selected in proportion to the area under the distribution for that integer.