Add a suggestion for how to resolve chunk manager not available error

[jsignell]

Just tries to add a suggestion about which library to install. @keewis had done something similar for guess_chunkmanager already.

For instance, if you have cubed but not cubed-xarray:

import cubed
import cubed.random

import xarray as xr

t_length = 50
spec = cubed.Spec(allowed_mem="2GB")

u = cubed.random.random((t_length, 1, 987, 1920), chunks=(10, 1, -1, -1), spec=spec)
v = cubed.random.random((t_length, 1, 987, 1920), chunks=(10, 1, -1, -1), spec=spec)

ds = xr.Dataset(
    dict(
        anom_u=(["time", "face", "j", "i"], u),
        anom_v=(["time", "face", "j", "i"], v),
    )
)
result = ds**2
result.compute()

[keewis]

looks good to me. I have one stylistic comment, and as mentioned during our chat today we need tests that triggers both suggestions.

For the unknown type we probably need a dummy (something that pretends to implement the relevant protocols, no need to be thorough), and for the known array type it should be fine to only run the test if dask is not installed.

[keewis]

thanks!