Add progressbar argument to_dataset_dict()

[andersy005]

This PR addresses #203 by adding progressbar argument to to_dataset_dict() method. This allows the user to override the default progressbar=True used during the class instantiation.

E.g.:

cat.to_dataset_dict(progressbar=False)

In [1]: import intake                                                                                              

In [2]: col_url = "https://raw.githubusercontent.com/NCAR/intake-esm-datastore/master/catalogs/pangeo-cmip6.json"  

In [3]: col = intake.open_esm_datastore(col_url)                                                                   

In [4]: cat = col.search(experiment_id=['historical', 'ssp585'], table_id='Oyr', variable_id='o2', grid_label='gn')                                                                          

In [5]:dset_dict = cat.to_dataset_dict(zarr_kwargs={'consolidated': True, 'decode_times': False})                 


--> The keys in the returned dictionary of datasets are constructed as follows:
        'activity_id.institution_id.source_id.experiment_id.table_id.grid_label'
             
--> There are 17 group(s)
Progress: |███████████████████████████████████████████████████████████████████████████████| 100.0% 

With progress bar disabled:

                                                                                                         
In [6]: dset_dict = cat.to_dataset_dict(zarr_kwargs={'consolidated': True, 'decode_times': False}, progressbar=False)

Cc @jbusecke

[jbusecke]

Fantastic! Thanks a lot @andersy005