Remove unit registry, use cf-xarray's or any other.

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Possibility of a meta context=="hydro" #44
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• CHANGELOG.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• Removes the unit registry declaration, instead use the one set as pint's "application registry". In most cases, this will be the one built by cf-xarray, but starting with xclim 0.55, once xclim is imported it will be xclim's.
• Patch all units functions for the two caveats of cf-xarray's registry:
  • A scalar Quantity.magnitude is a zero-dim array, but we expect a float. I cast them explicitly everywhere we handle this.
  • The cf unit formatter is created but not set as default by cf-xarray. I explicitly use it everywhere we cast to string.
  • Remove uses of pct for "percent" as this is not CF and not supported by cf-xarray.

As we still expect CF outputs, this is not completely agnostic : the unit registry used must come from cf-xarray. But it does allow to use the one from xclim.

Does this PR introduce a breaking change?

Shouldn't.

Other information:

Once Ouranosinc/xclim#2040 is merged a simple import xclim should activate xclim's registry. Then, using the hydro context would look like:

import xsdba
import xarray as xr
from xsdba.testing.utils import gosset
go = gosset()

hist = xr.open_dataset(go.fetch("sdba/CanESM2_1950-2100.nc")).pr.sel(time=slice('1980', '2010'))
ref = xr.open_dataset(go.fetch("sdba/ahccd_1950-2013.nc")).pr.sel(time=slice('1980', '2010'))

# Naively
QDM = xsdba.QuantileDeltaMapping.train(ref=ref, hist=hist)
# Fails with
# DimensionalityError: Cannot convert from 'kilogram / meter ** 2 / second' ([mass] / [length] ** 2 / [time]) to 'millimeter / day' ([length] / [time])

import xclim  # (as of 0.55) this modifies the unit registry _IN PLACE_
with xclim.core.units.units.context('hydro'):
     QDM = xsdba.QuantileDeltaMapping.train(ref=dref.pr.sel(time=slice('1980', '2010')), hist=dsim.pr.sel(time=slice('1980', '2010')))
# Works! Wow!

I will be adding a doc example soon.

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[coxipi]

Nice, a lot cleaner!

[coveralls]

coverage: 74.011% (+0.02%) from 73.991%
when pulling 191f0c9 on clean-units
into f4f234c on main.

[aulemahal]

Example added to the "usage" page!