api: optional recursion of transitive relations

[tiborsimko]

Background

Consider the following set of claims:

id claimant subject  predicate     object   certainty
-- -------- -------  ------------- -------- ---------
01 arXiv    arXiv1   is_variant_of  DOI1       0.8
02 INSPIRE  arXiv1   is_same_as    INSPIRE1    1.0
03 INSPIRE  INSPIRE1 is_same_as    bibcode1    1.0

The is_same_as predicate represents strong equality, is_variant_of weaker
equality. Both relations are transitive, meaning that if x R y and y R z, then x
R z.

Approach 1: iterative queries client-side

Imagine the following query:

GET /claims?type=bibcode&value=bibcode1

that basically asks "who does know anything about bibcode1"?

From the raw claim matching point of view, this query should return only the
claim number 03, which was inputted using bibcode1. After which the client could
then ask "who knows about INSPIRE1 then?", proceeding iteratively on
the client side as need be. This is not ideal as it can lead to many nested
queries.

Approach 2: resolving transitive relations server-side

Considering the fact that the sought-after relation is transitive, the
ClaimStore itself could attempt to resolve these recursions on the server side,
returning directly also the claim 01 that is joined by means of claim 02 to the
claim number 03.

The behaviour of the API could be parametrised via a new recurse option:

GET /claims?type=bibcode&value=bibcode1&recurse=true

that would be False by default, True on demand.

Optionally, the query can accept a certainty argument:

GET /claims?type=bibcode&value=bibcode1&recurse=true&certainty=0.9

which would include into recursing only those relations that have a certainty of
at least 0.9.

(see also #8)