Added nnls decoder solvers and tests.

- Added three non-negative least squares (nnls) solvers,
  which are based off the `scipy.optimize.nnls` method.
Small improvements to `utils`

- Added a function for plotting general data as an image with
  sensible axes
- Fixed a warning in `numpy.py:array`
Added firing rate estimators

- use either kernels or ISIs to estimate firing rates from spikes
- put these functions in a new file `utils/neurons`
- added some rough tests for these functions
Added adaptive LIF neurons

- made one `SimNeurons` object in the builder, rather than
  having `SimLIF`, `SimLIFRate`, etc.