allow efficient sub-xy tile reads for TIFF

[toloudis]

Use Case

User has a 6000x6000 TIFF file and wants to read a small crop area using dask or otherwise.

Currently in the Reader, we only chunk on whole XY planes. So to read a crop section one might have to load the whole XY plane anyway.

It would be great if we could reduce the dask chunk size when the file format permits reading from smaller sections. In this case (tiff), I'm sure this partly depends on how the data is organized in the tiff file itself.