nseg->ncomp for swc reader; set assign_groups=True

[michaeldeistler]

This PR suggests a few things:

1. Rename nseg to ncomp when branches are initialized: branch = jx.Branch(comp, ncomp=4)
2. Rename nseg to ncomp in jx.read_swc(..., ncomp=4)
3. Rename nseg to ncomp throughout the entire codebase internally
4. Set max_branch_len=None by default. We now have .set_ncomp(8) to deal with long branches.
5. Set assign_groups=True by default.

All of these things are also reflected in the SWC tutorial. I also advertise .set_ncomp in the SWC tutorial now.

[jnsbck]

I think we might want to move away from nseg altogether anyway, since if will be less confusing for the user. i.e. also change cumsum_nseg, jx.Branch(ncomps=2) etc. accordingly

Will take a look tmr.

[jnsbck]

My only high level comment is that we might wanna do this for all nseg. Otherwise good to go from my side.

Also, if we start working with deprecations now, we might wanna think about integrating this into the CI. See #516 for an idea of how to do this.