Adding anisotropic data

In order to add anisotropic data, a few things need to be taken into account:

• The resolution needs to be adapted, see https://git.embl.de/pape/covid-em/-/blob/master/stack_to_mmb_format.py#L81
• The chunks need to be adapted, see https://git.embl.de/pape/covid-em/-/blob/master/stack_to_mmb_format.py#L82
• The downscaling factors are currently hard-coded to values that make sense for isotropic data, see https://git.embl.de/pape/covid-em/-/blob/master/mmb_utils/data_export.py#L34. I would rather not expose these via the command line, because one needs to type in a lot of values. Either we use some heuristic to determine them, or we provide them in a separate json.

cc @schorb @hennies