Round to nearest integer for star polymer arms.

[pkienzle]

Fixes #298.

This is a breaking change. Preexisting models with non-integer number of arms will silently return a new value.

The choice of rounding means that the both end points (currently 1 and 6 arms) will be processed correctly.

The parameter arms can only be fit by an optimizer that can handle the step function, such as DREAM.

[pkienzle]

Note there is a little bit of inconsistency in the parameter names. Should be n_arms to match integer parameters in other models. Similarly num_pearls to n_pearls in the pearl necklace and linear pearls model.