Fixed model seeding (fixes #169).

- The model seed is now used to generate missing ensemble seeds.
- Some of @xchoo's inhibition stuff used gains that are now not available
  until build time, so this is now handled in the connection classes.
- Added some tests for the seeding (in test_model.py)
- Commented out longer circconv tests, since they were too slow.
Removed some properties from Ensemble.

- these properties were no longer necessary with the new build process.
- also removed model_seed argument from model.simulator
Removed fixed_seed from Model