Define slots explicitly

[bdestombe]

Describe the bug
Upon importing the library:

/srv/conda/envs/notebook/lib/python3.7/typing.py:845: FutureWarning: xarray subclass DataStore should explicitly define __slots__
  super().__init_subclass__(*args, **kwargs)

To Reproduce
Import the dtscalibration library

Expected behavior
No warnings.

Additional context
Xarray, does not recommend subclassing (and thus using slots), but instead to put everything under an accessor:

ds.dts.single_ended_calibration()

For reasons listed here: http://xarray.pydata.org/en/stable/internals.html#extending-xarray
Mostly needed when combining multiple 'extensions' to xarray. But to my opinion, this requires a lot of work, breaks the current version, and would not change the functionality of the package.

[BSchilperoort]

As for now I have decided to specifically silence this warning, as it just clutters the output and could confuse users. We might have to keep watching what xarray decides to do, but they seem to not have made any changes or decisions over the last year.

[BSchilperoort]

@bdestombe do we still need to tackle this issue? If our way of subclassing xarray is deprecated, it could break upon any new release of xarray. This might be a thing we should do before v3.0 😔

[bdestombe]

Hi Bart, this has been the case for several years now. Furthermore, I don't see any urgent deprecation warnings and therefor also don't think we have to tackle this now.

Instead, let's build an accessor in parallel to the current implementation.

[BSchilperoort]

I have been trying out accessors, and one problem is that it's difficult for code analyzers to determine what the type and available methods are of xarray/pandas extensions: microsoft/pylance-release#1112

In VS Code, the code analyzer does not see that regrid is a class under ds (Dataset), and that to_dataset is a valid function. No docstrings are displayed either:

[BSchilperoort]

Fixed by #204