adding amountNet and amountGross to value, updating value definition

[odscjen]

closes #817

[mrshll1001]

One very minor change in the form of a typo, otherwise what I'd expect based on the issue,.

[mrshll1001]

This looks fine, typo amended.

[jpmckinney]

We can omit "as a number" as this is repetitive with the type validation keyword.

I know "The amount as stated in the tender notice / award notice / contract notice document as a number" is the text I suggested in #817 (comment), but Value objects can appear in many different places, and the numbers might not originate in these specific documents in those cases. The number might also originate from an electronic system, such that there is no "document" per say.

Unfortunately, as an alternative, I don't think we can use turns of phrase like "original" / "unadjusted" / "unmodified" amount or similar, as we do allow the amount of Value objects to change over time, and it might just confuse readers as to the semantics.

Maybe something like "The amount as entered into a system or as published in a document."

This PR should also be supported by changes to the Markdown page for the Value object, to explain how the three fields ought to be used/interpreted (e.g. there are three fields so that, when it is unknown whether taxes are included or not, it can nonetheless be entered into amount – please be more eloquent than me :)).

The Markdown page should encourage publishers to describe their semantics for amount, e.g. a publish that has access to the amountNet and amountGross can have a policy of entering the amountNet in the amount. (We can perhaps also encourage this approach of using amount for amountNet when all are available, for consistency across datasets.)

[odscjen]

@jpmckinney I think all the requested changes to this PR were made so just bumping back for your final review

[jpmckinney]

Not sure why I left this PR open so long. I think I had paused my work on OCDS 1.2 around the time the updates were made.

This new description of amount resolves some of the ambiguities that I was maybe unsure about (e.g. how to interpret amount depending on which other fields were available).