Development of standalone xarray merge tool

[charlesbluca]

Looking through #6, it seems like we have the knowledge and infrastructure to create a standalone xarray merging tool that could work using information provided by aggregation_control to merge the assets of a catalog - this could be particularly useful in moving forward with Pangeo Forge, which would seek to automate the process of data merging/uploading to cloud.

@andersy005, what luck have you had implementing apply_aggregation in intake-esm?

cc @matt-long

[matt-long]

apply_aggregation is implemented and is generally functional. During the CMIP6 Hackthon, however, we encountered several issues, most related to anomalies in the underlying data. I hope that we can sort these out over the next couple of weeks.