Resurrect Lindblad Evolution and Add Sequential Propagation

[SRaent]

What

Ideally reinserts lindbladian evolution
Added sequential propagators that use less ram

Why

Linbladian evolution was in the code but not accessible.
Reduces code complexity and nested ifs.
Closes Issue #215

How

A new function in the Model takes care of returning Hamiltonians or Liouvillian superoperators according to model.lindbladian.
In this way propagation.pwc() basically just needs to do the exponentiation.

Remarks

Checks on signals (e. g. passing times) are supposed to be moved to the Generator and might be faulty now.

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

• 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 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
• 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

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ frosati1
❌ Moritz Schu

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[lgtm-com]

This pull request introduces 1 alert when merging 4f50fef into 003824c - view on LGTM.com

new alerts:

• 1 for Unused local variable

[codecov]

Codecov Report

Merging #221 (1d9a793) into dev (369f3d1) will decrease coverage by 0.77%.
The diff coverage is 50.84%.

❗ Current head 1d9a793 differs from pull request most recent head eaeb22a. Consider uploading reports for the commit eaeb22a to get more accurate results

@@            Coverage Diff             @@
##              dev     #221      +/-   ##
==========================================
- Coverage   74.79%   74.02%   -0.78%     
==========================================
  Files          38       38              
  Lines        5559     5597      +38     
==========================================
- Hits         4158     4143      -15     
- Misses       1401     1454      +53     

Impacted Files	Coverage Δ
c3/libraries/propagation.py	42.10% <13.33%> (-19.47%)	⬇️
c3/experiment.py	76.29% <70.00%> (-0.86%)	⬇️
c3/model.py	88.38% <92.85%> (+1.92%)	⬆️
c3/libraries/chip.py	62.63% <100.00%> (+0.84%)	⬆️
c3/optimizers/modellearning.py	89.86% <100.00%> (+0.04%)	⬆️
c3/optimizers/optimalcontrol.py	67.44% <100.00%> (ø)
c3/qiskit/c3_backend_utils.py	56.52% <0.00%> (-4.20%)	⬇️
c3/qiskit/c3_backend.py	91.11% <0.00%> (-2.64%)	⬇️
c3/parametermap.py	93.46% <0.00%> (-0.36%)	⬇️
... and 6 more

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[lgtm-com]

This pull request introduces 2 alerts when merging c6e109e into 003824c - view on LGTM.com

new alerts:

• 2 for Unused local variable

[lazyoracle]

@SRaent Can you add tests for the lindbladian evolution? In the future we would want to know if some work done elsewhere breaks this important functionality.