* 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.

- 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

At this low level a cell tree is just a sequence of parent indices,
where each parent index is the parent of the current index in the
sequence. There four basic algorithms:

1. child_count(parent_index):
   Computes the number of children for each node in parent_index
   	Time: O(N), Space: O(1)
2. branches(parent_index):
   Returns a set of the branching nodes in parent_index, last branch
   index equals the parent_index's size.
	 Time: O(N), Space: O(N)
3. expand_branches(branch_index):
   Takes a branch_index (result from branches()) and expands it at the
   size of the original tree with all the nodes inside a branch having
   the same number.
         Time: O(parent_index), Space: O(1)
4. make_parent_index(parent_index, branch_index):
   Return a compacted tree (single node per branch) from parent_index
   and its corresponding branch_index.
	Time: O(N), Space: O(N)

There is another utility function as well:

find_branch(branch_index, nid):
    Returns the id of the branch where nid belongs to.
	Time: O(N), Space: O(1)

merge to my fork

Remove redundant #ifdef'ed code

Address clang compiler warnings