Introduce `AbstractObservable` that represents the location of measurements that should be included in a observable.

[inmzhang]

Relates to #271.

I'm no sure whether the AbstractObservable representation is convenient for people working on circuits generation. But in principle, this simple representation should include all the information to derive a scalable observable that consists of the measurements.

AbstractObservable has two attributes:

1. top_lines: frozenset[Cube | Pipe]: the measurements along a line at the top of these blocks should be included in the observable.
2. bottom_regions: frozenset[Pipe]: the stabilizer measurements within a region at the bottom of these block should be included in the observable.

I did not explicitly include the measurement basis or line direction here, because they could be inferred easily from the block type.

Any opinions?

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[inmzhang]

Thank you for the review! I'll keep this PR open for a while to see if anyone who wants to work on the circuit generation part has questions about this representation. Enjoy your holidays!

[inmzhang]

I will be taking a two-week vacation to go traveling. During this period, I will keep this PR open.

[inmzhang]

Hi Adrien @nelimee, do you need to review this PR again before I merge it?

[nelimee]

Hi Adrien @nelimee, do you need to review this PR again before I merge it?

I will do a quick check again and tell you in less than 30 minutes :)