Opexebo

* Speedscore modified substantially
- Adaptive filtering added, allowing the user to specify an upper speed as a bandwidth
- Adaptive filtering adjusted to behave more sensibly in the case of small bin sizes
- Speeds are smoothed before correlating
* Spatial cross correlation added
* General `bin_number` to `bin_size` code refactored to make future development more consistent
* Assorted unit tests added, including run-once code removed for speed
* Fix errors associated with `>` where applied to MaskedArrays (which, unlike ndarrays, do not correctly obey the symbol)

* Ratemaps updated to handle walk filter (signature now requires providing animal speeds)
* Angular occupancy coverage calculation corrected
* circular arenas now handled in Spatial Occupancy
* Population Vector Correlation added

* Assorted bugfixes to HDtuning
* Gridscore to handle NaNs more gracefully

* `sep` made an optional dependency
* Alternative, non `sep` based code uses `skimage`
* Attempting to use `sep` if not installed with raise an error

* Consistent calculating of bin numbers when histogramming

* Third development release
* Improve NaN handling throughout by moving to MaskedArrays
* Fix Angular handling to function in seconds instead of frames
* Remove matplotlib from requirements