(re-)add schemata checks as soon as column type recognition is implemented

[patrikguempel]

With the upcoming merge of #123, two schemata checks are currently not implemented (anymore) in test_add_column and test_add_columns. The respective checks are commented out. This is, because at the time being, 'schema' does not track column types of empty columns correctly.
After this is fixed by #322, the above mentioned checks should simply be commented back in.

[Marsmaennchen221]

Blocked by #322

[lars-reimann]

Closing this, since we don't compute schemas ourselves anymore.