Add dry run feature (#151)

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...