Fix drag pulse helper method

[andrea-pasquale]

While testing https://github.com/qiboteam/qibocal/pull/689 I noticed that the platform's helper methods for creating drag pulses were broken (I was the one who implemented them...). This PR fixes the issue.

Checklist:

• [ ] 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:

Project coverage is 66.51%. Comparing base (aa1d165) to head (80907cf).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #835 +/- ## ========================================== + Coverage 66.35% 66.51% +0.16% ========================================== Files 50 50 Lines 6063 6063 ========================================== + Hits 4023 4033 +10 + Misses 2040 2030 -10 ``` | [Flag](https://app.codecov.io/gh/qiboteam/qibolab/pull/835/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/835/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | `66.51% <100.00%> (+0.16%)` | :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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