Define and optionally enforce structure of `number` values

[pudo]

Based on discussion with @tillprochaska in the context of https://aleph.discourse.group/t/ftm-proposal-optionally-specify-unit-for-number-properties/212/5

Let's define that a number is eventually meant to be a thing that contains only digits, can start with a - and at most one .. This way aleph can facet on it. Does this mess too much with the duration semantics from schema.org? The point is that we've never said this before, and so until now it's very much anything goes.

Next up: defining unit and making that into a well-defined concept - do we need to be able to do conversion?

[tillprochaska]

Let's define that a number is eventually meant to be a thing that contains only digits, can start with a - and at most one ..

What about allowing at most one , as a thousands operator? It’s not a deal-breaker, but especially when scraping data sources that contain formatted numbers, I could imagine that parsing them before passing them to FtM could be a little annoying. On the other hand, it could also conceal potential errors…

[pudo]

Interesting edge case we're running into: opensanctions/opensanctions#2423