sharing custom_checks

[loco-philippe]

@roll @peterdesmet

Two custom_checks were shared during discussions (issue #803 and PR #859):

• fields relationship: see first Notebook
• conditional constraints: see second Notebook

Is there a structure for sharing custom_checks or is it better to use a third party (e.g. Validata)?

[roll]

My opinion that custom_checks belongs more to the software level like frictionless-py or great-expectations. On the other hand after we clarified Extensions I think there might be a really good opportunity to create something like Tabular Quality extension