Improve test coverage

[phackstock]

Closes #309.

Changes implemented

Added tests

• Improved test coverage to:

Module	statements	missing	excluded	coverage
nomenclature/init.py	27	2	0	93%
nomenclature/countries.py	35	2	0	94%
nomenclature/codelist.py	261	7	0	97%
nomenclature/processor/region.py	305	8	0	97%
nomenclature/processor/required_data.py	119	3	0	97%
nomenclature/testing.py	62	2	0	97%
nomenclature/validation.py	35	1	0	97%
nomenclature/config.py	86	2	0	98%
nomenclature/definition.py	52	1	0	98%
nomenclature/code.py	109	1	0	99%
nomenclature/cli.py	34	0	0	100%
nomenclature/core.py	22	0	0	100%
nomenclature/error.py	15	0	0	100%
nomenclature/processor/init.py	3	0	0	100%
nomenclature/processor/meta.py	21	0	0	100%
nomenclature/processor/processor.py	6	0	0	100%
nomenclature/processor/utils.py	3	0	0	100%
tests/conftest.py	32	0	0	100%
tests/test_check_aggregate.py	26	0	0	100%
tests/test_cli.py	93	0	0	100%
tests/test_code.py	49	0	0	100%
tests/test_codelist.py	124	0	0	100%
tests/test_config.py	13	0	0	100%
tests/test_core.py	93	0	0	100%
tests/test_countries.py	14	0	0	100%
tests/test_definition.py	49	0	0	100%
tests/test_definition_variable.py	13	0	0	100%
tests/test_meta.py	24	0	0	100%
tests/test_model_registration_parser.py	14	0	0	100%
tests/test_region_aggregation.py	76	0	0	100%
tests/test_required_data.py	36	0	0	100%
tests/test_testing.py	24	0	0	100%
tests/test_validation.py	36	0	0	100%
Total	1911	29	0	98%

the remaining few percent I deemed either not necessary to test or point to potentially unused code which I didn't want to touch for this PR.

parse_model_registration

• Cleaned up the docstring and added an option to set a target directory for the generated model mapping and region list.
• Added a test.

Aligned CLI with standards

• Changed the list option from --dimensions [one, two] to --dimensions one --dimensions two. This is now in line with cli standards.

Made the setitem method of CodeList more robust

• Added checks if the item added is of type Code and if the key and the code name are identical
• Added tests

Clean up config

• Removed unnecessary code from NomenclatureConfig.
• Force validation of local_path
• Added tests

[danielhuppmann]

Just a minor suggestion in the docstring, otherwise good to go - and nice work!