Fix morphology section ctor bug.
Add morphology pools for miniapp from which morphologies are drawn in the recipe.
Add command-line options to expose above.
Add option --report-compartments to (slowly) check the min, mean, and max number of compartments in the generated cells across the simulation.
Use morphology::add_section to do all the heavy lifting; no need to make section_geometry objects by hand, unless you really, really want to.

Add a dry run mode, inspired by the dry run mode implemented in NEST. A dry run
of a model simulates running a large distributed model by running only the work
of one of the ranks, with artificial spike input from the other "dummy" ranks.

This is implemented as a new global communication back end, dryrun_global_policy,
the implementation of which is straightforward:

a new implementation of gather_spikes that takes the local spikes and
replicates them n times where n is the total number of simulated
ranks.
the global_policy::size() method returns the number of ranks in the
simulated run
the new back end has to store some state that records the number
of simulated ranks and cells per rank, which are set using the new
global_policy::set_sizes() method
Some CMake modificatins were required:

make the selection of the global communication backend have the same
interface as that for selecting the threading back end.
small improvements to the selection of the threa...

* 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.

* single file per rank output creates one file per rank
* global file output creates one file only from rank 0

* single flag -f that turns output on (off by default)
* other parameters have to be provided by an input file
    * if -f is set the defaults will be used

fix importing of file export parameters from json

Rename exporter_interface
Move exportor to io directory

* Reformat some line breaks.
* Remove blank lines after comments.
* Reword/improve some comments.
* Explain what CustomCmdLine does in more detail.
* Remove vestigial declaration of 'args' in
  `CustomCmdLine::reorder_arguments()`

* Address issue #74: missing json configuration options.
* Allow parameters from input file to be overriden on command line.
* Missing parameters in json input will defer to defaults.
* Add option for saving current options to json file (allowing
  for json parameter file format discovery.)
* Fix annoying command line argument reordering from TCLAP.
* Fix bug in assignment of `nothing` to a util::optional<T>
  variable.

Moved spike buffer to the manager. This allows reuse the by other registered exporters

Default is false
No output via command line parameters

Some compilers are more liberal than others…

New options:
 * -p, --probe-ratio      Attach probes to this proportion of cells
 * -X, --probe-soma-only  Don't probe dendrites
 * -P, --trace-prefix     Specify where to write traces
                          (default: 'trace_')
 * -T, --trace-max-gid    Don't trace cells with a larger gid than this.

Plus: coding guidelines compliance.

* -S (--syntype) option passes argument through to
  synapse creation in `make_cell`.

- improve robustness of time step and time integration
  interval selection
- changes of the type `if (!id)` to `if (id==0)`.
- fix bug selecting source cell gid when generating
  network connections
- make all-to-all network a user-specified option
    - updated README
- fix bug where an initial spike was not generated for
  the first cell on a communicator when the cell gid
  was a multiple of 20

* the user can now specify simulation time and time step size
    - via command line arguments
    - via json input file

* add header only library TCLAP for parsing command line arguments
    - added as git submodule
    - update CMakeLists to include headers
* remove the ring model from miniapp
* add support for user supplied parameters for miniapp
    - arguments for cell counts etc can be command line flags
    - or in a user-defined json input file
* the all-to-all model supports synapses-per-cell is less than
  the total number of cells
    - in this case a random set of sources is selected

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