Check whether a column is a Datetime without relying on default Datetime LogicalType

In checking whether a column is a valid time index, we check if `ltype_class == ww.logical_types.Datetime`, which means that if a user wanted to deregister the `Datetime` LogicalType and add in one of their own, they would not be able to make any columns that use that LogicalType a time index.

We should have another way of defining if a LogicalType is permissable for time indices.
- Maybe a `datetime` standard tag (not great, because it'll look weird with the Datetime Logical Type)