star polymer has an integer number of arms

[pkienzle]

Star polymer currently operates on a real-valued number of arms parameter, whereas the physical system should have a discrete number of arms. See discussion in #625.

Easy enough to truncate the value to the nearest integer, but special handling will be required for fitting. For now only DREAM will work. Bumps will need modifications to the other optimization methods to support integer parameters. [#256, https://github.com/bumps/bumps/issues/92]

Polydispersity could be supported on the number of arms. Distributions would need to be modified to treat integer parameters correctly and/or new integer distributions added.