* move from index_view to const_index_view for mechaninism
  inintialization. This allows us to pass more generic container
  types to the constructor while avoiding redundant copies
* NOTE: required update of
    * external/vector
    * external/modparser
* fixed signed-vs-unsigned comparison warning in algorithims.hpp

Note: use of non-const reference in `mechanism_catalogue::make`
is an interim solution.

* Fix bug in 'close' event delivery for multicell.
* Fix displacement scaling bug in gather_all_with_partition.
* Command line parameter io, reporting for new miniapp options.
* Only master rank performs command line parameter writing.
* Add test for mpi::gather_all and mpi::gather_all_with_partition.
* Extend utility of `util::assign`, `util::append`.

* Include new option -g in miniapp to specify cell group size.
* Miniapp -r option generates a ring network.
* Modify EXPECTS() so that when assertions are disabled we
  avoid unused variable warnings.

* Consolidate basic cell models used in unit and validation tests.
* Test `fvm_multicell` with same tests as `fvm_cell`, plus
  some index tests for the multiple cell case.

See defect report:
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1467

Note: failed assertion at run time with both fvm_cell
and fvm_multicell.

Bugfix/gcc4.9.3 compatibility

Latest Multicore Optimizations

Minor fixes for PR#86 branch.

* Count tests with int not bool in `mpi_listener.hpp`
* Ytpo fix in `gathered_vector.hpp`
* Add missing include in `test_algorithms.cpp`

* fix compile time error in src/model.hpp
* add mpi_listener type for sensible output and error reporting
  when using MPI with Google Test.
* add stub for communicator unit testing

Migrate cell_group and fvm_cell to internal API.

* Initialize voltage in `fvm_cell::initialize` to `resting_potential_`.
* Remove errant end-of-line whitespace.
* Fix merge conflict (merged master).

* fix build errors in tests with TBB
* set default probe output to none in miniapp

* the user can specify one of {none,AVX,AVX2,KNL} as
  targets for vectorization
* if modcc is available in PATH it will be used, which
  makes cross compilation easier
* a new flag USE_OPTIMIZED_KERNELS can be set to
  generate optimized kernels from the modcc compiler
  (equivalent to adding -O flag to modcc)

* `nest::mc::threading::sort` is analogous to `std::sort`
    * when using TBB the `tbb::parallel_sort` is used
    * otherwise `std::sort` is used
* used to parallelise connection construction during setup
    * now all initialization and simulation is multi-threaded
* added a unit test