Check how to find init state

[nwittler]

What

Find an appropriate initial state for sequence evaluation if none is explicitly provided.

Why

closes #189

How

Check for the thermal state initialisation task before attempting to use it. If it's not present, check for a model method.

Remarks

If the model has no method to provide the ground state this raises an error. We could discuss whether we require a model to provide the method or what custom error to raise instead.

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
• [x] Type Annotations - All new code has been type annotated in the function signatures using type hints
• [x] 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[bot]]

Codecov Report

Merging #190 (0b69bbb) into dev (7f08ff6) will increase coverage by 0.65%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #190      +/-   ##
==========================================
+ Coverage   73.28%   73.94%   +0.65%     
==========================================
  Files          37       37              
  Lines        5570     5572       +2     
==========================================
+ Hits         4082     4120      +38     
+ Misses       1488     1452      -36     

Impacted Files	Coverage Δ
c3/experiment.py	77.15% <100.00%> (+0.10%)	:arrow_up:
c3/model.py	86.45% <100.00%> (+1.66%)	:arrow_up:
c3/utils/tf_utils.py	64.59% <0.00%> (+0.62%)	:arrow_up:
c3/libraries/chip.py	61.79% <0.00%> (+4.11%)	:arrow_up:
c3/libraries/tasks.py	79.03% <0.00%> (+11.29%)	:arrow_up:

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