Return asset's attributes as keys when aggregate=False

[andersy005]

Addresses #163 (comment)

import intake
col_file = "https://raw.githubusercontent.com/NCAR/intake-esm-datastore/master/catalogs/pangeo-cmip6.json"

col = intake.open_esm_datastore(col_file)
query = dict(experiment_id='historical', table_id='Oyr', 
                 variable_id='o2', grid_label='gn', member_id='r1i1p1f1')
cat = col.search(**query)

# Disable aggregations
In [21]: dsets_pp = cat.to_dataset_dict(zarr_kwargs={"consolidated": True}, aggregate=False)                                                                                         
--> The keys in the returned dictionary of datasets are constructed as follows:
        'activity_id.institution_id.source_id.experiment_id.member_id.table_id.variable_id.grid_label'

--> There will be 2 group(s)

In [22]: dsets_pp.keys()                                                                                                                                                              
Out[22]: dict_keys(['CMIP.CCCma.CanESM5.historical.r1i1p1f1.Oyr.o2.gn', 'CMIP.IPSL.IPSL-CM6A-LR.historical.r1i1p1f1.Oyr.o2.gn'])

[andersy005]

Cc @matt-long