Standardize database source column

[jacksund]

Expand on the types of sources that may be needed:

And also add method to the DatabaseTable base class to enable grabbing these different sources:

    def get_source_seed(self):
        """
        WARNING: This is an experimental method.
        
        Attempts to jump back through sources until we find the orginal 'seed'
        source -- that is, a calculation that has some external source such as
        user input or a third-party database.
        """
        
        from simmate.toolkit import Structure
        
        current_source = self.source
        while current_source and "calculation_table" in current_source:
            current_source = Structure.from_database(current_source).calculation.source

        return current_source

    def get_source_children(self):
        """
        WARNING: This is an experimental method.
        
        Attempts to jump back through sources of all tables and find which ones
        point back to this given object as a seed source. It will return all
        child sources that meet this criterion.
        """
        raise NotImplementedError