[PoC] Add support for multiscale DataTrees

[alxthm]

These changes rely on xpublish-community/xpublish#313, which introduces the concept of DataTrees in xpublish.
With these changes, TilesPlugin can now serve tiles from a multiscale DataTree.

It is a proof of concept intended to support discussion for #172.

Among other limitations, this change does not support the entire multiscales specification yet
(https://github.com/zarr-conventions/multiscales). It assumes a simple DataTree layout, with each child corresponding to a tile matrix zoom level.

Example:

<xarray.DataTree 'pyramid'>
Group: /
├── Group: /0
│       Dimensions:  (lat: 8, lon: 16)
│       Coordinates:
│         * lat      (lat) float64 64B -85.0 -60.71 -36.43 -12.14 12.14 36.43 60.71 85.0
│         * lon      (lon) float64 128B -180.0 -157.5 -135.0 ... 112.5 135.0 157.5
│       Data variables:
│           foo      (lat, lon) float32 512B 0.0 0.0 0.0 0.0 0.0 ... 0.0 0.0 0.0 0.0 0.0
│       Attributes:
│           title:    Precomputed level 0
├── Group: /1
│       Dimensions:  (lat: 16, lon: 32)
│       Coordinates:
│         * lat      (lat) float64 128B -85.0 -73.67 -62.33 -51.0 ... 62.33 73.67 85.0
│         * lon      (lon) float64 256B -180.0 -168.8 -157.5 ... 146.2 157.5 168.8
│       Data variables:
│           foo      (lat, lon) float32 2kB 1.0 1.0 1.0 1.0 1.0 ... 1.0 1.0 1.0 1.0 1.0
│       Attributes:
│           title:    Precomputed level 1
└── Group: /2
        Dimensions:  (lat: 32, lon: 64)
        Coordinates:
          * lat      (lat) float64 256B -85.0 -79.52 -74.03 -68.55 ... 74.03 79.52 85.0
          * lon      (lon) float64 512B -180.0 -174.4 -168.8 ... 163.1 168.8 174.4
        Data variables:
            foo      (lat, lon) float32 8kB 2.0 2.0 2.0 2.0 2.0 ... 2.0 2.0 2.0 2.0 2.0
        Attributes:
            title:    Precomputed level 2