Make carrier attribute optional

[Bachibouzouk]

Description

I noticed that carrier was set at the component level and should be explicitly set at bus level in case of multi-input/multi-output components in rl-institut/oemof-tabular-plugins#26 otherwise it leads to logical problems that a component has one carrier assigned although it deals with multiple carriers. This is needed for book-keeping of energy flows from different sectors.

It is possible without any changes to assign a carrier column to the bus.csv resource, however it is not possible to not have a carrier column in resources based on Facades which have carrier as an attribute.

I experimented and saw that the fact that the carrier attribute is required in the definition of the facades. It throws an error at this line if I try to instantiate a facade without providing a value for the carrier attribute.

Fixes #oemof-tabular-plugins/issues/26

Type of change

Right now I outline two suggestions in the two commits for discussion purposes, I will then reset the commits and implement an approved solution.

Suggestion 1:
Always provide carrier as an empty string if "carrier" not found in the kwargs to the facade. This is working well and is a two-liner fix but it is not elegant.

Suggestion 2:
Make carrier attribute optional and with a default value. This is more elegant but we need to change that in all facades

Please tick or delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

Please tick or delete options that are not relevant.

• New and adjusted code is formatted using the pre-commit hooks
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
  Existing tests do not pass locally on latest dev but they pass online
• I have added new features/fixes to the CHANGELOG
• I have added my name to AUTHORS