cable_cell segment interface read-only (#889)

* Make access to `segment`s in a `cable_cell` read only
* Change cell constructor to require morphology and label dictionary; `make_cable_cell` no longer required.
* Consolidate paint/place functionality on `cable_cell`.
* Add simple cell builder wrapping new API for use in unit tests.
* Remove validation test code: appropriate validation tests will need to be implemented in nsuite.

Fixes #871