Updates to guidance and schema around missing information and exemptions

[ScatteredInk]

Placeholder, with more detail to follow, but missing info and exemptions don't work well in 0.1 and will need to be sorted out for 0.2.

In particular: where do reasons for missing information and exemption sit?

[ScatteredInk]

#149 also points to some earlier work on this that got left in to the 0.1 release.

[ScatteredInk]

I have adjusted the schema to allow for information on why information is missing to sit in entity and person statements, as well as in ownership-or-control statements.

The rationale for this is that there are times when you will know what a person or entity exists but can't/aren't allowed to identify it. The metadata on why that information is missing is useful for tracking enforcement, responsibility etc.

There are also cases where a missing entity/person could appear in the middle of the structure - and in those cases we want to be able to continue building a structure despite that certainty (which is already possible) but still say why we don't the information (which isn't possible).

I've added two simple person-based examples under examples/missing-data - but the same principle would apply for entities. I think we can add to these later.

The other change I've made is to add information-unknown-to-publisher as a reason for missing data - the current codelist is focused on cases where we know who is responsible for providing the data; this covers the case where someone is looking for the data and just can't find it.

[ScatteredInk]

These changes have now been made. I think there is a case for looking at different codelists at different places in the data structure for 0.3 release, so that the use of this is less ambiguous.

[ScatteredInk]

Some entities are expected to have no beneficial owners, e.g. a state has legal personality but has no BOs. This should be covered by the codelist.

[ScatteredInk]

Covered in #240 and #253