Use CATMAID stack info to create xarray.DataArrays

[clbarnes]

Introduces a Stack class which loads stack and mirror info from CATMAID. That has a get_scale method which returns an xarray.DataArray representing that scale level, which wraps a dask array wrapping a zarr array. Depending on the mirror info, the zarr array pulls either from an N5 store, or one of a few custom zarr store shims implementing a few of our more common tile source types (JPEG stacks, for now, but H2N5 should be fairly low-hanging fruit as well). xarray allows us to pull data in voxel or world space.

The Stack can be created simply from a catmaid client instance and stack ID or title (or by selection with a CLI). By default the JPEG tile sources re-use the session from that client instance (so that auth headers are re-used).

Using dask arrays means we can have concurrent downloads and also do things like lazy transposes, which means we can use CATMAID's orientations. However, there is quite a lot of complexity in the interactions between N5, Zarr, and CATMAID orientations, which needs a fair bit of testing.

To do

• Literally any testing at all 😬
• HTTP auth for tile stores
• HTTP auth for N5FSStore
• Make sure the orientations all work out correctly
• Docs
• Maybe implement some more tile sources