152 basic implementation mapper

[S-Linde]

This Pull Request includes:

• Abstract Base Class for the mapping part of the Mapper pass
• An IdentityMapper. Mapping the first virtual qubit to the first physical qubit, etc.
• An HardcodedMapper. Map using a predefined (hardcoded) mapping.
• Standardized tests for the Mapper class. Checks barebones of what we require of a Mapper.
• Use the output of the Mapper class in a mapper pass on the Circuit object.

This also includes the following bugfix:

• __eq__ of Measure no longer returns an array of bools.

Closes #152

[elenbaasc]

Really nice work!

Mostly comments on phrasing things in the docstrings.

The _QubitReIndexer in the general_composer module seems to do something similar to the relabel methods you added in the squirrel_ir module. Do you think we should refactor either one of those? I.e. recycle the QubitReIndexer for the relabel methods or remove the _QubitReIndexer and rely also apply the relabel methods there?

[S-Linde]

How do I activate the github actions. When I made this pull-request the github actions started and some of them failed. Then (I think) I fixed the problem, but now the github actions are not running. What do I need to do to let them run?

[elenbaasc]

A new commit to this branch should initiate the GitHub actions. Otherwise something is not configured correctly....