Enable basic testing of models

First approach to scalable testing of all models

All 24 tests are now working in CI Testing time: ~15 minutes

Changes summary:

Parametrize and simplify tests (code have been reduced by 50% and allows easy expandability)
Added auto download of model dependencies if they are missing. Note: This is for further discussion
Added retry of model package download (sometimes one request from Zenodo wasn't enough and model was not downloaded)
Reduced the number of samples generated in tests to speed up testing. Now, it takes about 15 minutes to run all tests (including 4 models)
Included global.json file in the repo, I am not sure about the location, but I think it is a better idea to have it in the medigan repository
fixed package requirements for pix2pix model, several libraries were missing