Handling NaN nodata values in reclassify_raster

[phargogh]

This small PR handles the case where a raster passed to pygeoprocessing.reclassify_raster has a NaN nodata value. I chose to use pygeoprocessing.array_equals_nodata instead of simply adding equals_nan=True to numpy.isclose so that we could take advantage of the special case speedups that are handled in that helper function.

Dave and Claire, I requested a review from both of you because you were discussing this over in #454.

Fixes:#454

Readthedocs build failures are addressed in #470

[claire-simpson]

Looks good to me, thanks @phargogh!

[davemfish]

Thanks, @phargogh