Exclude defaults for yaml export

[phackstock]

closes #231.

Fixed the issue with a couple of additional changes

Additional changes implemented

• Code and CodeList now feature __eq__ methods which makes for easier comparisons for tests. For the equality operator of Code I explicitly exclude the file property as it does not influence functionality.
• Code.file is now a named attribute.
• Streamlined a few unit test accordingly.

Open question

There is one case which is currently not covered very nicely which concerns the create_yaml_from_xlsx function.
Say we have an excel like this:

Variable	Description	Unit	Region-aggregation
Primary Energy	Total primary energy consumption	EJ/yr	[{"Primary Energy (mean)": {"method": "mean"}}]
Primary Energy (mean)	Mean primary energy consumption	EJ/yr
Share|Coal	Share of coal in the total primary energy mix

this will be translated into the following yaml:

- Primary Energy:
    description: Total primary energy consumption
    unit: EJ/yr
    region-aggregation:
      - Primary Energy (mean):
          method: mean
- Primary Energy (mean):
    description: Mean primary energy consumption
    unit: EJ/yr
    region-aggregation:
- Share|Coal:
    description: Share of coal in the total primary energy mix
    unit:
    region-aggregation:

The issue is that Primary Energy (mean) and Share|Coal both feature an empty region-aggregation attribute since the column exists in the excel and the values for those two variables are None.
I don't think that there is a way from distinguishing this unintentional None from the intentional one for Share|Coal unit.
Functionality wise this does not change anything though.

I don't think that there's much we can do here, just wanted to point it out.

[danielhuppmann]

Nice, thank you!

[danielhuppmann]

I don't think that there is a way from distinguishing this unintentional None from the intentional one for Share|Coal unit.

Don't think that this is an issue, but thanks for thinking it through carefully!