Make the soma a separate CV (#816)

Addresses issues #814 and #815.
* Change the discretization so that a spherical soma comprises its own CV.
* Modify the `div_compartment_integrator` to accommodate the single CV soma, adjusting the discretization of the most proximal branches, adding a half-size CV attached to the soma, so as to leave the remainder of the discretization the same.
* Rename the confusing `ncomp` to `ncv` inside the discretization code.