Introduce add_update methods into DatabaseMapping

[manuelma]

So the idea is to provide methods that receive an item spec and then add that item if it doesn't exist, or update the current if it exists.

Then, import_data needs to be refactored using these methods - should be very easy and actually the initial definition of the add_update method can be copied directly from the current import_data.

Also, we need to clarify in the documentation of the import_functions module that it is an alternative to add_update that is a bit more compact and convenient in some cases.

I am not sure if we want to start deprecating import_data after this, seems a bit inbuilt into things like the JSON format that is also used for SpineOpt templates. Maybe we can consider updating that JSON format and the templates to work with add_update instead? Anyways I think this last point is for further discussion.

[soininen]

I am not sure if we want to start deprecating import_data after this,

True, you have some good points there. I think we are not in a hurry to deprecate import_functions in any case. Maybe the JSON import could be its own function if we do not have one yet (I have not seen one in spinedb_api but I may have just missed something).

[manuelma]

Done.