Marker pulse qblox backend

Explanation of changes

A MarkerPulse is defined in the pulse_library. For Qblox hardware it is implemented via the set_mrk Q1ASM command. For this purpose a new io_mode called "digital" is defined for the Sequencer object.

Motivation of changes

Users need to be able to easily specify digital pulses with exact timing to drive third-party electronics during experiments.

---

Merge checklist

See also merge request guidelines

• Merge request has been reviewed (in-depth by a knowledgeable contributor), and is approved by a project maintainer.
• New code is covered by unit tests (or N/A).
• New code is documented and docstrings use numpydoc format (or N/A).
• New functionality: considered making private instead of extending public API (or N/A).
• Public API changed: added @deprecated (or N/A).
• Tested on hardware (or N/A).
• CHANGELOG.md and AUTHORS.md have been updated (or N/A).
• Windows tests in CI pipeline pass (manually triggered by maintainers before merging).

---

For reference, the issues workflow is described in the contribution guidelines.