Ouranosinc / xclim / 12875104937

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Clean and export the units registry (#2040)

### What kind of change does this PR introduce?
* Remove support for non-CF aliases : "cms" (m3 s-1), "mmday" (mm d-1)
and "pct" (%). These are not supported by `udunits2` and thus I don't
see why we should support them ? @huard, @RondeauG what do you think ?
I'm not strongly opinionated on this.

* Set the unit registry as pint's "application registry" to make it
usable by non-xclim aware applications. To be obtained with `ureg =
pint.get_application_registry()`.

* Units of quantities printed in indicator attributes are now formatted
using the CF syntax (ex: "mm d-1" instead of "mm/d"). Not sure how I
missed that earlier...

* Clean up in the code of the registry declaration and addition of
comments to explain what is happening.

* A small change in `dataflags` that was actually unneeded at the end,
but it felt cleaner to me, so I kept it.

### Does this PR introduce a breaking change?
Some units are not supported anymore (`cms`, `pct` and `mmday`).
Some attributes will be different as the units are printed differently
(for fractions mostly).

### Other information:
The registry export allows an application/module that is units-agnostic
in itself, to be used with xclim. For those applications / usages, the
import order might be significant though : `xclim` has to be the last
`set_application_registry` done. Specifically in the climate case, as
`xclim.core.units` imports `cf_xarray.units` (which also sets the app
registry), a subsequent `import cf_xarray.units` is not problematic as
python will _not_ actually execute it.

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