Party role

[ColinMaudry]

Adds the partyRole code list (open-contracting-extensions/european-union#65).

I took the definition of Central purchasing body from Article 2 1. 16) of directive 2014/24/EU.

I figured out the definition of "Process contact point" from the context.

The other definitions are either copied or derived from those mentioned in the issue`.

[ColinMaudry]

Is it OK to have commits from the merged #2? Or shall I cancel this PR and do a new one?

[jpmckinney]

I think you can do:

git checkout partyRole
git merge master
git push

Then, the old commits will disappear from the PR.

[jpmckinney]

For all descriptions, can you add articles, e.g.:

• The body
• A procuring entity
• A contact point
• The service
• A party that