Added some more decoder methods.

- Added some L1/sparse solvers
- decoder methods that solve for a weight matrix are now supported
Cleaned up decoder solvers and 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.
Moved `sample_hypersphere` to utils.builder

- 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`.