Use integer number of branches for star polymer

[pkienzle]

from ticket #815 comment 6 by SMK:

Looking at Higgins & Benoit (as I don't have access to the reference given for the star model) I suspect that arm is a relatively modern term and that branch was the original terminology.

That being so, I find it difficult to comprehend how you can have a non-integer number of branches.

The branches may be of different length (cf molecular weight, degree of polymerisation), but that is a different issue.

So I would vote that we make arms into n_arms.

[butler]

actually the star polymer model requires not only integer number of arms but that the arm lengths are all equal and that all branches originate from a common point (hence star). In other words the distance between branches on the backbone has to be zero.