Dry-run mode (#582)

Dry-run mode: 
* An implementation of distributed_context that is used to mimic the performance of running an MPI distributed simulation with n ranks.
* Verifiable against an MPI run with the same parameters. 

Implementation: 
* Describe the model on a single domain (tile) and translate it to however many domains we want to mimic using arb::tile and arb::symmetric_recipe. This allows us to know the exact behavior of the entire system by only running the simulation on a single node.
* Mimic communication between domains using arb::dry_run_context

Example: 
* dryrun in example/ is a verifiable example of using dry-run mode with mc_cells

Other:
* Documentation of dry-run mode 
* unit test for dry_run_context