Explore `value_type` errors

[ehwenk]

There are 2 seemingly unrelated errors all related to value_type that probably have a common source:

• dataset_tests throws an odd value_type error for all datasets
• the methods table currently has value_type.x and value_type.y, indicating it has been added to this table twice

For a while I was wondering if the methods table shouldn't have value_type at all - but thinking that for now I'll leave it in the table once - and therefore also in schema

[yangsophieee]

The order of value_type and other columns in the methods table seems to change when I run test_build_dataset on different test datasets. Not sure if this is a real issue though but we might want a fixed order to the methods table columns.

[ehwenk]

closed with 8714247