Qblox backend custom functionality for QD spin qubit experiments

Explanation of changes

Adds workaround for generating (effectively) arbitrarily long DC square pulses, a 'chirp' pulse and a staircase.

MR can be broken down in two (related) parts:

• New pulses: added two new pulses to the pulse library, along with a function for them to evaluate.

  • Chirp: Sinusoid with frequency that changes linearly as function of time
  • Staircase: Ramps from one value to another in finite discrete steps

• Qblox backend: Implements special treatment for the staircase and square pulses (at baseband) to allow them to be much longer than the waveform memory of the Pulsars can accommodate.

Motivation of changes

Certain experiments done for QD spin qubits require DC signals longer than the waveform memory of the pulsars can hold. These special pulses are treated separately by the backend by generating them programmatically from the sequencer program instead.

To allow this new pulses were necessary, hence their inclusion in this MR.

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Merge checklist

See also merge request guidelines

• Merge request has been reviewed and approved by a project maintainer.
• Merge request contains a clear description of the proposed changes and the issue it addresses.
• Merge request made onto appropriate branch (develop for most MRs).
• New code is fully tested.
• New code is documented and docstrings use numpydoc format.
• Changelog has been updated (when applicable).
• CI pipelines pass
  • black code-formatting passes (gitlab-ci),
  • test suite passes (gitlab-ci),
  • no degradation in code-coverage (codacy),
  • no (serious) new pylint code quality issues introduced (codacy),
  • documentation builds successfully (CI and readthedocs).