Virtual variables

[abkfenris]

In XREDS @mpiannucci implemented a cool pattern to support dynamic generation of new xr.DataArrays in a dataset based on the existing data.

He is using it it to generate Mean Lower Low Water levels based off of NAVD88 in his datasets, and an attribute pointing to the VDatum offsets: https://github.com/asascience-open/xreds/blob/main/xreds/extensions/vdatum.py

To do so he created his own hookspec DatasetExtensionSpec.transform_dataset which he calls in his dataset provider https://github.com/asascience-open/xreds/blob/main/xreds/dataset_extension.py

I had been thinking around the same idea for a while, and then @srstsavage brought it up in the XREDS repo to support generating virtual vector variables. asascience-open/xreds#28

I think we should standardize this pattern into a new type of plugin.

[abkfenris]

Also we can support the flat earthers the -180-180/0-360º problem.