Add keys_info() method

[andersy005]

Change Summary

This PR adds a .keys_info() method as requested by @matt-long in #461. It returns a dataframe in which the keys are index and the columns are the groupby attributes.

In [1]: import intake

In [2]: cat = intake.open_esm_datastore("./tests/sample-catalogs/cesm1-lens-netcdf.json")

In [3]: cat.keys_info()
Out[3]: 
                component experiment stream
key                                        
ocn.20C.pop.h         ocn        20C  pop.h
ocn.CTRL.pop.h        ocn       CTRL  pop.h
ocn.RCP85.pop.h       ocn      RCP85  pop.h

Related issue number

• Closes Add method to return information about catalog.keys() #461

Checklist

• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable