PCRL handles unknown users and/or items

[vlainic]

Description

I have changed recom_pcrl.py following the example of recom_mcf.py, as they have the same modality. I run a test locally on my data and it worked fine.

I did this following #444 as I could not understand why PCRL does not handle this, while all others I have tried to. You made all models just raise ScoreException and then make it a default score. After reading the PCRL paper, I could not understand why it couldn't handle unknown users or items as any other model here. TBH, I think that all models with modalities should somehow handle cold-start, even with the global mean.

Related Issues

As I mentioned above, this is related to the Issue #444 .

Checklist:

• [-] I have added tests.
  • I think there is no need to add a test.
• [-] I have updated the documentation accordingly.
  • Same here, as handling unknown parameters is not documented anywhere :)
• [/] I have updated README.md (if you are adding a new model).
• [/] I have updated examples/README.md (if you are adding a new example).
• [/] I have updated datasets/README.md (if you are adding a new dataset).