Collect duplicate code errors

[phackstock]

Closes #104, closes #312.

Instead of reporting duplicate code errors one by one we now collect them and raise them as one single error. For better readability the individual errors are indented and numbered.
From previously reporting:

ValueError: Duplicate item in variable codelist: Secondary Energy|Liquids

we now have:

ValueError: Collected 529 errors:
  1. Duplicate item in variable codelist: Secondary Energy|Liquids
  2. Duplicate item in variable codelist: Secondary Energy|Liquids|Biomass
  3. Duplicate item in variable codelist: Secondary Energy|Liquids|Electricity
  4. Duplicate item in variable codelist: Secondary Energy|Liquids|Other
  5. Duplicate item in variable codelist: Secondary Energy|Solids
  6. Duplicate item in variable codelist: Secondary Energy|Solids|Biomass
  7. Duplicate item in variable codelist: Secondary Energy|Solids|Fossil
  8. ...

Update 24.01.2024 10:09

Since IAMconsortium/legacy-definitions#1 has already been resolved, I had to also address #311.
The current way the test test_multiple_external_repos is configured is not the most elegant way but it should be ok.
It tests for the following:

1. Are the two external repos correctly parsed from the yaml file
2. Are the two external repos correctly fetched from GitHub
3. Is the overall size of the resulting variable list (common + legacy definitions) bigger than 2000 entries.

The reason I opted for 3. instead of testing for the presence of specific variables is that we expect to migrate variables from legacy to common definitions. This way it could happen that at some point all the variables that I would test for to prove that both legacy and common definitions are imported are now wholly in common-definitions.
That's why I opted to check for the overall size of the code list. Also not 100% future proof but good enough for now I'd say.

[danielhuppmann]

Looks good, thanks!