Added some other decoder methods
Added some more decoder methods.

- decoder methods that solve for a weight matrix are now supported
WIP: added L1 weights solver
WIP: added some more sparse decoder solvers
Can now specify weight solvers for connections
Added allclose function to tests/helpers.py

- allows you to account for delays, exclude the beginning
  and end of signal, and do a plot.
- also added filtfilt function for delay-free filtering.
Adding early version of test_decoders to save my work
Cleaned up decoder solvers and related tests.

- also updated `allclose` helper function