Check if constituents of common regions are in native regions

[dc-almeida]

Closes #443
Constituent regions used for region aggregation should also be native regions, otherwise, unexpected errors may appear (as caught in this PR.

[dc-almeida]

The two tests failing are caused by the common-definitions hash pointing to a version where the error found here remains:

nomenclature/tests/data/region_processing/external_repo_test/nomenclature.yaml

Lines 4 to 7 in 941fc73

	repositories:
	common-definitions:
	url: https://github.com/IAMconsortium/common-definitions.git/
	hash: cb85704

Safe to update hash, @danielhuppmann?

[phackstock]

@dc-almeida, yes please go ahead and update the hash.

[phackstock]

Two smaller changes would be good to implement.

In addition, technically, the solution implemented does not cover all cases.
Something like this, albeit exotic, should still work:

model: model_a
common_regions:
  - region_a
  - region_b
exclude_regions:
  - region_c
common_regions:
  - common_region_1:
    - region_a
    - region_b
    - region_c

Currently, I think it would raise an error because it does not find region_c in common_regions.
I've never really seen a use case like that though so, I'd say it's fine for now. What do you think @danielhuppmann?

[phackstock]

Just one small suggestion, after that, good to merge (without the need for another review)