Changes to new API, 2drepresentation example works

- New API can make filtered probes now
- Old API connections fixed (dimensionality issues)
- Added ability to serialize model to JSON and Pickle,
  very useful for looking at models, but loading not implemented yet.
Lots of unit test changes

- Added helpers to run tests in multiple simulators
- Old API tests extracted out to test_node and test_ensemble
  (work in progress)
- Removed some empty test files
- Plotting simplified; plots should be saved to simulator-specific
  folders, not shown (added a context manager for this in helpers)