to_yaml does not indent lists

[phackstock]

Discovered during 218, reading a yaml file into a CodeList object and dumping it out to yaml again does not work for nested structures.

As an example a variable code list with a region-aggregation attributes:

- Price|Carbon:
    definition: Price of carbon (as reported by the model)
    unit: US$2010/t CO2
    region-aggregation:
        - Price|Carbon (mean):
            method: mean

reading it in and writing it out to yaml again:

VariableCodeList.from_directory("variable", "definitions").to_yaml()

gives the following:

Price|Carbon:
    description: Price of carbon (as reported by the model)
    unit: US$2010/t CO2
    region_aggregation:
    - Price|Carbon (mean):
        method: mean

The indentation under region_aggregation is missing which breaks reading the yaml file.

It seems that this is a known issue with pyyaml (yaml/pyyaml#234 and https://stackoverflow.com/questions/25108581/python-yaml-dump-bad-indentation) so it shouldn't be too hard to fix.

[phackstock]

Closed in #218.