Import mappings from external repo

[phackstock]

closes #267.

Changes

• There is now a new configuration class called NomenclatureConfig. Example:

repositories:
  common-definitions:
    url: https://github.com/IAMconsortium/common-definitions.git/
definitions:
  region:
    repository: common-definitions
  variable:
    repository: common-definitions
mappings:
  repository: common-definitions

• In analogy to the readthedocs configuration this configuration must be in a file called .nomenclature.yaml located in the root of the repository.

Open questions

• I'm open for suggestions on the name of the configuration class and the configuration file. I just had to put something there for now.
• With the option now to instantiate CodeList and RegionProcessor objects entirely from and external repository, the from_directory methods of both classes no longer accurately reflect their functions.
• Additionally, the folders for definitions is currently required to exist, even if it's empty.

Maybe the last two points are better addressed in a follow-up PR though.

[danielhuppmann]

Looks good overall, a few minor comments and suggestions inline.

[danielhuppmann]

Looks good, thanks @phackstock!