Restless measurements data processing nodes

[catornow]

Summary

This PR implements the new data processing nodes RestlessNode and RestlessToCounts. These allow users to perform and analyze experiments based on restless measurements (without qubit reset) to significantly speed-up calibration experiments compared to standard experiments based on active reset, see https://arxiv.org/pdf/2202.06981.pdf.

Details and comments

See code. This PR implements the core part of PR #675.

[nkanazawa1989]

Thanks for splitting PRs. This is much easier to review. I added several comments.

[eggerdj]

This looks good to me a few minor comments on docs and tests.

[nkanazawa1989]

I added minor comment but rest of code looks good. Thanks @catornow

[nkanazawa1989]

Can we rename circuit_first to memory_allocation taking value "C" (circuit-first), "S" (shot-first) and show a both data structures in class documentation with these labels? Currently only circuit first is shown and user may get wrong impression that the node only supports circuit-first. This memory representation convention would be useful when we implement another type of data processor, i.e. such label can be attached to metadata.

[eggerdj]

Sounds good. We could make this convention more explicit with an Enum to add proper documentation. Perhaps:

from enum import Enum

class ShotOrder(Enum):
    """Description ..."""
    circuit_first = "c"
    shot_first = "s"

and then

    def __init__(self, validate: bool = True, memory_allocation: ShotOrder = ShotOrder.circuit_first):

[catornow]

Thanks, I implemented the code here: 07e777f.

[nkanazawa1989]

Looks good to me. Thanks @catornow! It would be great if you could write documentation for shot-first, but this doesn't block merging.

[eggerdj]

LGTM!