Updated xarray breaks dscim

[brews]

Dependabot tried to bump xarray 2022.11.0 to 2023.7.0 in #101 and it failed tests in CI.

Here are the archived logs from that CI run in case the link expires: logs_381.zip

I haven't dug through this but should fix the package so it can run on the latest versions of xarray.

[brews]

This is partly because binary operations respect keep_attrs as of xarray v2023.2.0 (see pydata/xarray#7391)

This means that the binary operation to calculate the anomaly here can cause later problems when the anomaly shares attrs keys with data it will be combined with here.

Keeping old behavior means ditching attrs during those binary operations, but maybe we should consider how xr.combine_by_coords handles attrs between the data it's combining. ...So, set something sensible for combine_attrs.

[brews]

Another problem seems to be that as of xarray v2023.3.0 bool arrays used for indexing can't be dask-backed. They need to be computed and in memory so their size can be comprehended.

This is a problem in lines like this.