Optimize Import for large dataset

[ponceta]

We want the user to be able to import large datasets from INTERLIS 2020_1 to TEKSI.

[sjib]

It takes very long when deselecting a class on the import wizard form (up to 5 or even 10 minutes). I can only deselect one class at a time

[domi4484]

At the moment all to be imported objects are stored in a sql alchemy session which can be seen as a sort of transaction. When we deselect/select a class in the import dialog, all objects are removed/added to that session and this is a slow operation.
The solution to that in my opinion is to rewrite the dialog and import procedure so that all objects are in memory only at the beginning. By accepting the dialog only the selected object/classes are added to the session and committed to the working schema.