Runtime distributed context (#485)

Move from choosing the distributed communication model from a compile time choice (the old `arb::communication::communication_policy` type) to a run time decision.

* Add `arb::distributed_context` class that provides the required interface for distributed communication implementations, using type-erasure to provide value semantics.
* Add two implementations for the distributed context: `arb::mpi_context` and `arb::local_context`.
* Allow distribution over a user-supplied MPI communicator by providing it as an argument to `arb::mpi_context`.
* Add `mpi_error` exception type to wrap MPI errors.
* Move contents of the `arb::communication` namespace to the `arb` namespace.
* Add preprocessor for-each utility `ARB_PP_FOREACH`.
* Rewrite all examples and tests to use the new distributed context interface.
* Add documentation for distributed context class and semantics, and update documentation for load balancer and simulation classes accordingly.

Fixes #472