Allow changing ncomp after initialization

[michaeldeistler]

This PR is a follow-up to #418 and #426. It allows to set nseg after initialization of the module. This is achieved 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, in which case it reuses the xyzr to compute optional radiuses:

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

[jnsbck]

just noticed that comb_branches_in_each_level is now obsolete. Could you rm it in this PR as well? :)

[michaeldeistler]

Will do!

[michaeldeistler]

Replaced by #436