Implement regrid operation based on datashader

[philippjfr]

In holoviz/datashader#375 a fast regridding implementation with no additional dependencies was merged into datashader. Since we already provide an aggregate operation we should simply add a regrid operation to the operation.datashader module in HoloViews, which will allow for much faster regridding of Image types than the current approach which expands the Image to a large table and then runs aggregation on it. I have already written an operation based on the original gridtools library the datashader implementation was based on, which should be easy to adapt to use the datashader API instead.

[philippjfr]

This is now waiting on various issues in datashader to be addressed (holoviz/datashader#380), I've written the operation now but there are a number of showstoppers before I can continue.

[philippjfr]

Reassigning to 1.9 since this relies on changes in datashader.

[philippjfr]

The regrid operation has now been implemented.

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