Add zarr v2 endpoints to Tiled

[genematx]

This PR exposes Tiled data as a zarr collection on a set of new api endpoints, /zarr/v2/.... This allows one to use zarr clients directly with Tiled, as if it was an external filesystem accessed through fsspec.

Assuming a demo Tiled server is running on 127.0.0.1:8000 (e.g. started with tiled serve demo), one can read its contents into zarr by first specifying a file system mapper and then passing it to zarr:

import zarr
from fsspec import get_mapper

url = "http://localhost:8000/zarr/v2/"
fs_mapper = get_mapper(url)
root = zarr.open(fs_mapper, mode="r")

The resulting object is a zarr.Group, which represents the root of the Tiled catalog tree and supports (most) of the usual operations on zarr groups:

>>> print(group)
<zarr.hierarchy.Group '/' read-only>

>>> list(group.keys())
['dynamic', 'flat_array', 'high_entropy', 'low_entropy',
'nested', 'scalars', 'structured_data', 'tables']

>>> root.tree()
/
├── dynamic (3, 3) float64
 ├── flat_array (100,) float64
 ├── high_entropy (100, 100) int64
 ├── low_entropy (100, 100) int32
 ├── nested
 │   ├── cubes
 │   │   ├── tiny_cube (50, 50, 50) float64
 │   │   └── tiny_hypercube (50, 50, 50, 50, 50) float64
 │   ├── images
 │   │   ├── big_image (10000, 10000) float64
 │   │   ├── medium_image (1000, 1000) float64
 │   │   ├── small_image (300, 300) float64
 │   │   └── tiny_image (50, 50) float64
 │   └── sparse_image (100, 100) float64
 ├── scalars
 │   ├── e_arr (1,) <U7
 │   ├── fortytwo () int64
 │   ├── fsc () <U5
 │   └── pi () float64
 ├── structured_data
 │   ├── pets
 │   └── xarray_dataset
 │       ├── lat (2, 2) float64
 │       ├── lon (2, 2) float64
 │       ├── precipitation (2, 2, 3) float64
 │       ├── temperature (2, 2, 3) float64
 │       └── time (3,) datetime64[ns]
 └── tables
     ├── long_table
     │   ├── A (100000,) float64
     │   ├── B (100000,) float64
     │   └── C (100000,) float64
     ├── short_table
     │   ├── A (100,) uint8
     │   ├── B (100,) uint8
     │   └── C (100,) uint8
     └── wide_table
         ├── A (10,) float64
         ├── B (10,) float64
         ├── C (10,) float64
         ...
         ├── X (10,) float64
         ├── Y (10,) float64
         └── Z (10,) float64

NOTE: To access Tiled servers that require authentication, we can pass an api-key in the header of the HTTP requests. With fsspec, this is done by explicitly constructing an HTTPFileSystem object and mapping it to zarr:

from fsspec.implementations.http import HTTPFileSystem

headers = {"Authorization": "Apikey your-api-key-goes-here",
           "Content-Type": "application/json"}
fs = HTTPFileSystem(client_kwargs={"headers": headers})
root = zarr.open(fs.get_mapper(url), mode="r")

The native tiled datastructures are mapped to zarr as follows:

Tiled	zarr
Container	Group
Array	Array
Sparse Array	Array (dense)
Data Frame	Group (of columns)
Data Frame Column	Array

Addresses the Issue #562.

Checklist

• Add a Changelog entry
• Add the ticket number which this PR closes to the comment section

[danielballan]

I cannot find where this is used. Maybe this was needed in some transient state of the PR but no longer is needed.

$ git grep aiohttp
pyproject.toml:    "aiohttp",
pyproject.toml:    "aiohttp",
pyproject.toml:    "aiohttp",

[danielballan]

Moreover, since we use starlette for the server and httpx for the client, it would be somewhat odd and redundant to use aiohttp as well.

[genematx]

It is used by fsspec.implementations.http.HTTPFileSystem, which is needed to connect to a tiled server that requires authentication. I had the same thought yesterday, that this was something I used before but no longer need, but unfortunately it's not the case. We don't need it in all requirements though (only for testing), which I have fixed now.

[danielballan]

Attempting to write raises a helpful error message:

ReadOnlyError: object is read-only

This behavior should be tested.

[genematx]

Added a couple of test cases here. Just to note, those errors are raised by fsspec and zarr client objects, even before the request reaches Tiled.