mapping-sheet: Add option to skip check for implicitly required fields

[duncandewhurst]

The project level schema in OC for Infrastructure doesn't permit null values, since it doesn't use the releases and records mechanism.

As such the mapping-sheet script marks all the fields as required, since it checks for fields which are implicitly required in the OCDS schema (i.e. where null is not a permitted value).

As a temporary workaround I've created an infrastructure branch of ocdskit which only checks for the explicitly declared required fields.

[jpmckinney]

I think we can just add a command-line argument to disable that behavior, instead of having a branch.

[duncandewhurst]

Agreed, this was the quick fix to get the mapping template working correctly and to share with @yolile

[jpmckinney]

There's now an --infer-required option, that needs to be used when working with OCDS schema.