Set different nseg

[michaeldeistler]

Follow-up to #418 and #426. This PR will allow to set nseg after initialization with a new set_ncomp method. This PR is the final requirement for a new release with these features.

Works for branches:

comp = jx.Compartment()
branch = jx.Branch(comp, nseg=2)
branch.set_ncomp(4)

...for branches within a cell:

comp = jx.Compartment()
branch = jx.Branch(comp, nseg=2)
cell = jx.Cell(branch, parents=[-1, 0, 0])
cell.branch(1).set_ncomp(4)

...and for branches read from SWC files (by actually reusing the xyzr to compute optional radiuses:

cell = jx.read_swc(fname, nseg=2)
cell.branch(1).set_ncomp(4)