Decoders

- I've added a number of new decoder solving methods (mostly for solving for sparser decoders or weight matrices)
- there are basic tests for these solvers, as well as benchmarks looking at performance as well as the optimal level of regularization
- `weight_solver` can be specified on connections to solve for weight matrices instead of decoders
- there are a few new utilities
  * `allclose` is for doing a fancier version of `np.allclose` with plots
  * `filtfilt` is for doing low-pass filtering with no phase shift (delay)