Qiskit with PWC gates

[lazyoracle]

What

Example usage of qiskit interface with pulse gates

Why

Closes #205

How

• Updated the Piecewise_constant_controls.ipynb notebook to include a section on usage with qiskit interface.
• Better error handling for unsupported gates or SetParamsGate not at the end
• Some cleanup of dead code

Remarks

The intended workflow for this feature is calibration type tasks where the piecewise constant description of a gate (or set of gates) is updated in every iteration of the calibration optimizer. E.g., with ORBIT, the circuits can remain as they are and the pwc description of the gates in the circuit can be updated by including a SetParamsGate at the end of the circuit with the modified pwc values for all the gate(s) being calibrated.

Checklist

Please include and complete the following checklist. Your Pull Request is (in most cases) not ready for review until the following have been completed. You can create a draft PR while you are still completing the checklist. Check the Contribution Guidelines for more details. You can mark an item as complete with the - [x] prefix

• [x] Tests - Added unit tests for new code, regression tests for bugs and updated the integration tests if required
• [x] Formatting & Linting - black and flake8 have been used to ensure styling guidelines are met
• [ ] Type Annotations - All new code has been type annotated in the function signatures using type hints
• [ ] Docstrings - Docstrings have been provided for functions in the numpydoc style
• [ ] Documentation - The tutorial style documentation has been updated to explain changes & new features
• [x] Notebooks - Example notebooks have been updated to incorporate changes and new features
• [ ] Changelog - A short note on this PR has been added to the Upcoming Release section

[codecov[bot]]

Codecov Report

Merging #218 (1eb51fc) into dev (5d6701d) will increase coverage by 0.42%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #218      +/-   ##
==========================================
+ Coverage   74.30%   74.72%   +0.42%     
==========================================
  Files          38       38              
  Lines        5545     5516      -29     
==========================================
+ Hits         4120     4122       +2     
+ Misses       1425     1394      -31     

Impacted Files	Coverage Δ
c3/qiskit/c3_backend.py	91.11% <100.00%> (+0.20%)	:arrow_up:
c3/qiskit/c3_backend_utils.py	56.52% <100.00%> (+29.73%)	:arrow_up:

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