add scan method

[phil20686]

We need to add a method to the engines to provide a list of the meta data objects for all stored items meeting a certain designation. The api is that this should be a function on IPersistenceEngine with the api:

def scan(name : str, params: {}) -> List[DatasetMetaDataStub]:

and where, eg, an entry in params of foo=3 means it would return only metadata stubs where parameter foo equaled three.