experiments: add inspire_classification instead of class number

[MJedr]

• INSPIR-3603

[michamos]

In theory this is an incompatible change, so the major version should be bumped, but this should affect only inspire-dojson (for experiments) and inspirehep itself (where we lock packages), so I think it's ok if we bump only the minor version (with sem-ver: new feature in the commit message), otherwise we'll need to bump a lot of other packages (hepcrawl, query-parser, matcher, merger, etc.). WDYT @drjova ?

[drjova]

@michamos sounds good