New propagation library created

[frosati1]

What: propagation library created
How: moved code related only with propagation from tf.utils to propagation.py in libraries
Why: cleaner code

[CLAassistant]

All committers have signed the CLA.

[lazyoracle]

Add some tests to the updated propagation.py library, following the basic tests currently present in our test suite. We can iterate on top of it.

[lazyoracle]

Related: https://github.com/shaimach/c3po/issues/49

[codecov]

Codecov Report

Merging #99 (a2c46f7) into dev (8369d14) will increase coverage by 6.09%.
The diff coverage is 27.41%.

@@            Coverage Diff             @@
##              dev      #99      +/-   ##
==========================================
+ Coverage   51.48%   57.57%   +6.09%     
==========================================
  Files          36       36              
  Lines        5409     5304     -105     
==========================================
+ Hits         2785     3054     +269     
+ Misses       2624     2250     -374     

Impacted Files	Coverage Δ
c3/utils/tf_utils.py	48.23% <ø> (+13.06%)	⬆️
c3/libraries/propagation.py	26.22% <26.22%> (ø)
c3/experiment.py	62.27% <100.00%> (+3.64%)	⬆️
c3/libraries/fidelities.py	21.65% <100.00%> (+0.25%)	⬆️
c3/optimizers/c1.py	73.73% <0.00%> (-7.43%)	⬇️
c3/libraries/chip.py	45.41% <0.00%> (-0.20%)	⬇️
c3/utils/parsers.py
c3/generator/devices.py	64.83% <0.00%> (+0.18%)	⬆️
c3/utils/utils.py	65.04% <0.00%> (+0.40%)	⬆️
... and 11 more

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