Opentheso READ connector

Kinda non RESTful API, but potentially interesting usecases.
Resources:

• https://opentheso.huma-num.fr/index.xhtml
• https://opentheso.huma-num.fr/swagger-ui/index.html
• https://pactols.frantiq.fr/swagger-ui/index.html
• https://pactols.frantiq.fr/index.xhtml

idk if there are Opentheso instances other than PACTOLS

changed milestone to %Connectors

added To Do up-for-grabs labels

assigned to @rwitz

WIP connector is available here:
https://gitlab.huma-num.fr/datasphere/heimdall/connectors/opentheso

Not released yet, because although it seems to work on PACTOLS, it's not toroughly tested, in particular on generic opentheso.huma-num.fr thesauri, which seem to use a lot more non standard URLs for narrower, broader and related, instead of honest to god ARKs or "same instance" URLs.

added To Validate label and removed To Do label