Potentially remove Region validators

[phackstock]

Since the source data file is coming from the EU I'd say it's safe to assume there aren't any mistakes in there. Therefore, technically this code:

https://github.com/iiasa/pysquirrel/blob/5d8005389d78f42a65863c3b175f879684d58643/src/pysquirrel/core.py#L37-L89

is not required.

I'm not quite sure if we should keep it or remove it.

Reasons to keep:

• Good to double check our input values regardless where they come from
• The code is nicely done

Reasons to remove:

• It makes the code longer than necessary an harder to read

What do you think @dc-almeida and @danielhuppmann?

[danielhuppmann]

If there's no harm and if it's nicely implemented, I'd say let's keep it. Maybe people can use it when developing similar packages for other world regions.

[phackstock]

Closing for now as we're keeping the validators.