Bonsai-python

Changed
- Simplify pytest settings.

Added
- Add more user friendly exception messages when encountering errors when
converting states from python dictionaries to protobuf messages.
- Add ability to pass command line arguments directly to the
method `run_for_training_or_prediction` as `*argv`. This is useful when
running a simulator from within another process.

Changed
- Allow protobuf versions greater then 3.1.
- Anonymous protobuf descriptors (descriptors that have no name) were being
assigned a randomly generated name. Now they are all assigned the same
static name.

Changed
- Update the way protobuf message classes are created from DescriptorProto
messages to be compatible with versions of protobuf beyond 3.1.

Fixed
- Fix a bug that was causing the `--recording-file` command line argument
to be ignored.

Changed
- Change test framework from nose2 to pytest.

Added
- Add support for simulator ids. The Bonsai platform now supports connecting
multiple simulators when training a brain. This support requires all
simulators be assigned a random id. The `sim_id` field has been added to the
protobuf message protocol as part of this change.
- Add a preliminary, alternate version of a Simulator interface. The new
classes in support of this are in the file brain.py. This code is a very
early, initial implementation and is not recommended for use.

Fixed
- Fix a memory leak when reconstituting protobuf classes by adding a
cache to the protobuf message builder.

Removed
- Remove the `selection` field from the `SimulationSourceData` protobuf
message, as it is no longer used.