Added two new decoder optimization routines.

- they may be faster in some circumstances
Renamed pytest `benchmark` option to `benchmarks`
Added heuristic to choose the number of eval points.

- fixes #290
Added tests for new optimization routines.
Added benchmarks for number of eval points
Added check for empty activities matrix
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
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 adapting LIF neurons

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