Fix deserialization of crosstalk_matrix

[andrea-pasquale]

This PR fixes the crosstalk_matrix deserialization which was broken after switching from yaml to json. I found this bug while testing https://github.com/qiboteam/qibocal/pull/684. If someone knows a more elegant solution let me know.

Checklist:

• [x] Reviewers confirm new code works as expected.
• [x] Tests are passing.
• [x] Coverage does not decrease.
• [x] Documentation is updated.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (1c8650b) 65.10% compared to head (e717b9f) 65.13%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #813 +/- ## ========================================== + Coverage 65.10% 65.13% +0.02% ========================================== Files 50 50 Lines 6027 6031 +4 ========================================== + Hits 3924 3928 +4 Misses 2103 2103 ``` | [Flag](https://app.codecov.io/gh/qiboteam/qibolab/pull/813/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/qiboteam/qibolab/pull/813/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | `65.13% <100.00%> (+0.02%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam#carryforward-flags-in-the-pull-request-comment) to find out more.

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[alecandido]

If someone knows a more elegant solution let me know.

I know: stop using non-integer IDs for serialization and move the matrix out of the single qubit :P