Move current clamps to shared state. (#1407)

* Change implementation of current clamp from a builtin mechanism to
  functionality provided by the back-end shared state classes.
* Extend clamp functionality to cover more use cases: it now supports DC
  and AC stimuli governed by a piecewise-linear envelope.
* Update Python iclamp wrapper to suit; add unit test.
* Account for total applied stimulus currents separately, and remove
  their contribution to the reported transmembrane current and
  ionic (i.e. non-capacitive) transmembrane current.
* Add new probe specifically for tracking total current stimulus in a cell.
* Update docs, and add an entry for current clamps to the concepts doc.
* Remove entirely the builtin catalogue.