Feature/spike handling

* change `probe_sort` enum to scoped enum
    - renamed to `probeKind`
    - refactored out of class
    - updated coding guidelines wiki with enum rules
* refactor `std::pair` into structs with meaningfull name types in `cell.hpp`
    - not just probes: stimulii and detectors too.
* add profiling region around sampling in `cell_group`
* change output data format for traces to json
* remove white space at end of lines (looking at you Sam)

Feature/probes

Fix linking of MPI libraries for tests and validation.

* Finds an actual (possibly) reasonable use for std::endl.

Sidesteps NDEBUG versus assert() issue; EXPECTS macros are
now enabled via a CMake configuration option -DWITH_ASSERTIONS=ON,
corresponding to the preprocessor define -DWITH_ASSERTIONS in
the code itself.

(Note, with assertions enabled, one of the existing tests in
test_algorithms.cpp aborts.)

- profiler generates one report per MPI rank
- estimates the thread imbalance per rank
- generates a json file with profiling stats for each rank
- works with/without MPI
- works with/without TBB

Conflicts:
	miniapp/miniapp.cpp

- move definition of `global_policy` to header file
  src/communication/global_policy.hpp
- the `#ifdef WITH_MPI` guards go there so that
    - they don't pollute user code
    - they are available throughout the application
- small cleanup of a couple of style points in the mpi code

* Make global_policy methods static
* Move global_policy init/finalize into static methods
* Make miniapp use only global_policy methods for
  setup, rank, etc.

* 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

Feature/spike handling

* Add WITH_MPI CMake option to enable MPI code in application
* Use find_package(MPI) and set up flags, etc. for MPI
* Extra defines to disable the inclusion of C++ MPI headers for
  MPICH and OpenMPI.
* Fix signed/unsigned compare warning in algorithms.hpp

Added MPI support to the miniapp
- added ring model and all-to-all model
- started refactoring general model setup steps into a model class in
  miniapp
- optional -DWITH_MPI to turn on MPI support

Low-level algorithms for manipulating cell trees.

Feature/optional review