Extend the ring benchmark to have an optional number of synapses attached to each cell, instead of a fixed count of one synapse per cell.
This doesn't change the behavior of the model: only the first synapse is used for communication. The other synapses only effect is to
increase the per-cell computational overheads, to more effectively mimic real world performance.

Make the execution context presented to users an opaque handle, moving all implementation of the gpu, thread and distributed contexts into the back end.

* move `execution_context` and `distributed_context` definitions to the back end
* create `execution_context` handle called `context` in the public API
* provide `make_context` helper functions that build different context configurations (default, user-specified local resources, with MPI)
* update documentation for all parts of the public API that touch contexts
* move `distributed_context` docs to the developer documentation (from the public API docs)

Benjamin Cumming authored 6 years ago and Sam Yates committed 6 years ago

* Add new ring benchmark to examples.
* Refactored common functionality for reading miniapp parameters from a json file to `aux` (used by both bench and ring).

Fixes #516.