Implement exact sampling for cable cells. (#1046)

Fixes #1037. 

* Add fake unmatchable spike events to event queue for each exact sample request, so that integration boundaries align with exact sample times.
* Update probe-demo for exact sampling.
* Add copy assignment for `cable_cell`.
* Extend max args for ARB_PP_FOREACH to 20.
* Add unit test for ARB_PP_FOREACH.
* Use ARB_PP_FOREACH to instantiate backend-specific probe unit tests. This also fixes the GPU probe tests that were using multicore backend.
* Add unit test that exercises exact sampling requests combined with regular events in a gap junction context.