suggested addition to Database or Schema class

[pavlis]

I think we need a new method for either Database or Schema that answers this question: For this data object what is the default collection used for saving one of these?

For reference, here is the (excessively complicated in my view) way it is handled in Database.save_data:

        schema = self.metadata_schema
        if isinstance(mspass_object, TimeSeries):
            save_schema = schema.TimeSeries
        else:
            save_schema = schema.Seismogram

        # should define wf_collection here because if the mspass_object is dead
        if collection:
            wf_collection_name = collection
        else:
            # This returns a string that is the collection name for this atomic data type
            # A weird construct
            wf_collection_name = save_schema.collection("_id")
        wf_collection = self[wf_collection_name]

What we need, I think, is a robust method that would have this signature (there may be a better, less verbose name):

def default_collection_name(self,mspass_data_object)->str:

It would need to throw an exception if the input were not one of the known types. It should handle both atomic data and ensembles. Typical use would be this:

# d is the datum to be tested
colname = db.default_collection_name(d)
dbcol = db[colname]

I think the implementation is a variant of the fragment from save_data.