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)

Remove redundant #ifdef'ed code

Address clang compiler warnings

* Add file cmake/CompilerOptions.cmake for setting up compiler
  specific options
* Disable 'missing-braces' warning on Clang
* Avoid defect in g++ 4.9.2 standard library that omits move
  constructor for `stdd::ifstream`
* Remove signed/unsigned warning in test_optional.cpp

Clang (by default) objects to the omission of parentheses in
expressios such as a && b || c, and also to brace elision
in aggregate initialisation. The latter complicates initialisation
of e.g. std::array. GCC has not turned this warning on for
that reason, and there is an open issue for this for clang:
https://llvm.org/bugs/show_bug.cgi?id=21629

* Add parentheses to resolve logical parentheses warning
* Suppress missing-braces diagnostic in test_optional.cpp

Use optional type for event_queue

* Use `#pragma once` instead of install guards
* Use `using` instead of `typedef`

* Move optionalm headers into src/util
* Change namespace for optional classes to nest::mc::util
* Include unit tests for optional and uninitialized classes
* Simplify (dedoxygenate) comments in uninitialized.hpp

Update README.md build instructions

Replace use of `std::pair<bool,T>` with `optional<T>` in
event queue code, where `optional` is taken from the
implementation defined in `external/optionalm`.

Basic synapse support

Better support tests in out-of-tree build.

Add path to data directory by preprocessor definition so that
unit tests can be run from any directory.

Synapses

This is a big and unweildy update (the last one, we promise).
- added synapses
    - update to a version of modparser that generates net_receive and
      point process currents
    - added event_queue type for queueing events in a cell
    - added advance_to() method to FVM cell that supports event delivery
    - added synapse storage and modification to cell and fvm_cell types
    - added unit tests and NEURON validation test
- include support for const_views
    - moved to more recent bcumming/vector library with support for
      const_views
    - added const-safe const_view interace to fvm_cell and matrix types

Merge Issue 18: Cleanup of includes

(yes, I'm one of 'those' programmers)

Use "" for local includes
Use <> for includes from include directories

Add all directories with files to include to CMakeLists.txt
Make all linux folder based includes global (remove the folder structure)

Use externalproject_add to manage mod->hpp depenedencies.

Builds modcc from modparser as an external project;
executable will reside under BUILD/external/bin where
BUILD is the CMake build directory.

CMake hackery (see 'build_all_mods' target) should generate
required module .hpp files from .mod files before the
cellalgo library is built, if the header files are
missing or out of date.

This should also suffice for performing out-of-source
builds.