Use xfail instead of silently passing tests

[jgosmann]

Motivation and context: This makes it more clear when certain features are not supported by an algebra.

To be able to use xfail, this adds a mechanism to declare what sidedness values are supported by specfic algebra operations. This mechanism is a decorator which adds an attribute with the allowed sidedness values to the method object.

Closes #281.

Interactions with other PRs: none

How has this been tested? existing tests

How long should this take to review?

• Average (neither quick nor lengthy)

Types of changes:

• New feature (non-breaking change which adds functionality)

Checklist:

• [x] I have read the CONTRIBUTING.rst document.
• [x] I have updated the documentation accordingly.
• [x] I have included a changelog entry.
• [n/a] I have added tests to cover my changes.
• [x] I have run the test suite locally and all tests passed.

[jgosmann]

@bjkomer, @arvoelke Does anyone have time to review this soon? Then it could be included in 1.3 release. (No worries if not, then it will be released later. It is barely a user-facing change anyways.)