Added some more decoder methods.

- Added some L1/sparse solvers
- Decoder methods that solve for a weight matrix are now supported
- Added tests
- Added allclose function, allows you to account for delays,
  exclude the beginning and end of signal, and do a plot.
- Added filtfilt function for delay-free filtering.
- Also added some documentation related to decoders.
Added logger messages for failed imports
Added some additional base solvers
Moved `filtfilt` to `utils/numpy.py`.
Removed commented code in `decoders.py`.