Add support for packing grid value dimensions

[philippjfr]

This PR makes it possible to construct gridded datasets where all the value dimensions are packed into a single array. This is often a much more natural format, e.g. for an RGB:

hv.RGB(([1, 2, 3], [1, 2, 3], np.random.rand(3, 3, 3)))
# or equivalently
hv.RGB({'x': [1, 2, 3], 'y': [1, 2, 3], ('R', 'G', 'B'): np.random.rand(3, 3, 3)})

Also adds support for packed value dimensions on xr.DataArray types:

da = xr.DataArray(data=np.random.randint(0, 255, (2, 2, 4)), dims=('x', 'y', 'band'), coords={'x': [1, 2], 'y': [1, 2], 'band': [0, 1, 2, 3]})
hv.RGB(da)

Closes #550
Closes #3311

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