Enforce mapping argument on associative memories

[jgosmann]

Motivation and context:
As discussed here this makes the mapping argument for associative memories required. This makes explicit which vectors are cleaned up and should lead to less surprise that vectors added later to a vocabulary are not included. It also ensures that in the GUI the vectors are added to the vocabulary instead of potentially creating an empty cleanup memory.

Furthermore, as syntactic sugar sequences of strings are allowed as syntactic sugar for {k: k for k in sequence}.

Interactions with other PRs:
based on #171 (that one provides pure bugfixes, this one provides API changes which require a minor release instead of a bugfix release)

How has this been tested?
added tests

How long should this take to review?

• Average (neither quick nor lengthy)

Types of changes:

• Breaking change (fix or feature that causes existing functionality to change)

Checklist:

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