Add Stan ContentTypes

[UlrikeS91]

from issue #7

Resources:

• https://mc-stan.org/ (webpage)
• https://search.kg.ebrains.eu/instances/615edae7-2697-49cf-b730-5b02bb652969 (KG model)

Info:

• "platform for statistical modeling and high-performance statistical computation"
• accessed through different interfaces: Command-line, R, Python (https://mc-stan.org/users/documentation/)
• at least for R, users are recommended to save their model files as ".stan" (as far as I understand, those are still "just" normal R scripts; https://mc-stan.org/docs/2_19/stan-users-guide/file-extensions.html)

Instances:
We could add the .stan as "application_vnd.stan.R" or "application_vnd.R.stand". I couldn't find much information about the others. Also, it seems to me that the KG model is just mentioning this as an option for running the model. How would this even be linked on the model?