Bugfix in transmon class

[alex-simm]

What

The Transmon class is using the anharmonic part of the Hamiltonian (Hs["anhar"]) which is, however, only generated if the levels are >2. A transmon with 2 levels throws an error. This fixes it.

Why

Because it wasn't working

How

Only use the parameter if levels are >2.

Remarks

Checklist

• Tests - Added unit tests for new code, regression tests for bugs and updated the integration tests if required
• 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 hint
• Docstrings - Docstrings have been provided for functions in the numpydoc style
• Documentation - The tutorial style documentation has been updated to explain changes & new features
• 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]

Codecov Report

Merging #146 (388a9af) into dev (2b4625f) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #146      +/-   ##
==========================================
- Coverage   71.43%   71.42%   -0.01%     
==========================================
  Files          36       36              
  Lines        5219     5218       -1     
==========================================
- Hits         3728     3727       -1     
  Misses       1491     1491              

Impacted Files	Coverage Δ
c3/libraries/chip.py	57.67% <100.00%> (-0.08%)	⬇️

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[nwittler]

Looks good.