Add method to return information about catalog.keys()

[matt-long]

The keys returned by catalog.keys() point to unique datasets in an intake-esm catalog (if queries are constructed using only the catalog.groupby_attrs). I have found it useful in some circumstances to get the values of the fields used to construct the keys; rather than a parsing the key string itself, I think this merits a method and wonder whether this should be part of intake-esm.

Here's a prototype implementation of such a method.

def get_keys_info(cat):
    """return a dictionary with the values of components of the keys in an
    intake catalog

    Example:
    key_info = {
        "20C.ocn.pop.h.1": {
            "experiment": "20C",
            "component": "ocn",
            "stream": "pop.h",
            "member_id": 1,
        },
    }

    """
    # generate a list of lists with all possible values of each groupby_attr
    # this is already being done in intake-esm
    iterables = [cat.unique(columns=key)[key]['values'] for key in cat.groupby_attrs]

    # generate a dictionary of keys with the values of its attributes
    key_info = {}
    for values in product(*iterables):
        key = cat.sep.join([str(v) for v in values])
        if key in cat.keys():
            key_info[key] = {k: values[i] for i, k in enumerate(cat.groupby_attrs)}
    return key_info