Add test for checking Example Notebooks

[lazyoracle]

Example Notebooks must be checked for every PR. The notebooks are typically the first point of entry for anyone using the code-base and it is not uncommon for the notebooks to go out of date (and become malfunctioning) when some breaking changes are introduced in the code.

[codecov-io]

Codecov Report

Merging #20 (591c7a8) into dev (bc2f66a) will decrease coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev      #20      +/-   ##
==========================================
- Coverage   46.95%   46.81%   -0.15%     
==========================================
  Files          35       35              
  Lines        4453     4398      -55     
==========================================
- Hits         2091     2059      -32     
+ Misses       2362     2339      -23     

Impacted Files	Coverage Δ
c3/libraries/envelopes.py	27.77% <0.00%> (-9.58%)	⬇️
c3/libraries/hamiltonians.py	86.48% <0.00%> (-2.41%)	⬇️
c3/libraries/fidelities.py	18.03% <0.00%> (-0.26%)	⬇️
c3/utils/tf_utils.py	44.20% <0.00%> (-0.24%)	⬇️
c3/libraries/sampling.py	0.00% <0.00%> (ø)
c3/libraries/estimators.py	0.00% <0.00%> (ø)

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[lazyoracle]

The Notebook Test check fails (expectedly) because of #19. Putting this PR on hold until that is resolved.