Option to plot nrn_secid labels

[arnaudon]

This adds NEURON section id labels to each section corresponding to the NEURON id of the given section list.

It can be used as follow:

neurom view --plane xy --nrn-secid [all|axon|basal|apical] my_morph.asc

If the provided name is not a valid section list, it will fall back to use all.

[arnaudon]

So I will move this to morph-tool asap, and close this one afterwards.