* make a new gathered_vector type for storing a "distributed" vector
  that has been gathered onto a single node
    * stores the vector
    * stores the partition information
* split communicator::exchange() into two phases
    * exchange() which simply exchanges spikes
    * make_event_queues() which generates local queues from spikes
* now callbacks for both local and global spike store can be
  performed in the model
* and extra meta data about the partition is available for more
  advanced parallel global storage schemes.

Global and Local spike exchange streamline

Exporter fixes

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

* MPI ranks now test the file that they generate, not that rank 0
generates
* code formatting updates
* added interface for getting file file name used by exporter

* 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

Exporter fixes

Feature/exporter spike file

- sorting of includes
- bracket location
- all arguments on the same hight
- use of auto in the performance/io/disk_io.cpp

Do some small textual changes

fix importing of file export parameters from json

Rename exporter_interface
Move exportor to io directory

Merge branch 'master' of https://github.com/eth-cscs/cell_algorithms into feature/exporter_spike_file

Improve miniapp command line argument handling

* Resolves some conflicts.

Add 'nop_function' helper for trivial functions.

* 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()`

Increased the output size

Save values in a array, to prevent changes on the heap

Remove the flush