Correctly deal with sparse resolutions

[philippotto]

For example, a dataset which has the following resolutions [[4, 4, 2]] should be handled correctly. I'd defer this until this is requested by users, though.

[philippotto]

Today, we got two requests for that so we should think more about it.

A very quick "solution" would be to extrapolate the resolutions array so that [[4, 4 ,2], [16, 16, 16]] is padded to [[1, 1, 1], [2, 2, 1], [4, 4, 2], [8, 8, 4], [16, 16, 16]] or similar.

A more elegant way would probably be to change the rendering code to always do resolution lookups based on exponents instead of indices (so, zoomstep 2 would be mapped to index 0 for a [[4, 4 ,2], [16, 16, 16]]-dataset). However, here we would lose the ability to convert positions into arbitrary zoomsteps (which is quite handy for computations).