Voltage and current probes

* First implementation of simple current/voltage probe
  interface
* Sample a specific collection of voltage or current
  values at independent dt (current implementation
  chooses to provide at most one sample per simulation
  dt per request.)
* Miniapp uses samplers to write soma and dendrite
  membrane potentials from first three cells.
* Also: fix noncompliant use of nullptr -> bool conversion
  in test_optional.cpp