refac: introduce consistent convention for linopy operations with subsets and supersets

[FabianHofmann]

Related to #550 #571

Changes proposed in this Pull Request

Establish a consistent coordinate alignment when linopy objects interact with DataArrays that have subset or superset coordinates

TODO:

• extend test suit to linopy + linopy objects
• manifest convention in the docs
• clarify relation to linopy.align
• really ensure convention holds true for all combinations of addition/multiplication/comparison + subset/superset` + order + object types
• come up with a convention for equally shaped objects Fix coordinate misalignment in expression merge #550

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.