test: check shape of kinematics dataset

[redeboer]

The doc CI for #192 fails because the shape of the data set that is returned by HelicityKinematics.convert is incorrect: its values contain floats, and should all be lists. This PR adds a test that checks the shape.

[redeboer]

@spflueger The test will fail here too. Strange though that a dataset has the correct format in the example notebook, but that the one in the tests contains float values (not wrapped in a list).

[redeboer]

Note: I assume the shape of a data set (a dict of ndarrays) to be such that it can be directly converted into a pandas.DataFrame, like happens here:
https://pwa.readthedocs.io/projects/tensorwaves/en/0.1.3/usage/2_generate_data.html#visualize-kinematic-variables

[codecov]

Codecov Report

Merging #194 (eec0bfa) into master (6eebdf5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #194   +/-   ##
=======================================
  Coverage   82.17%   82.17%           
=======================================
  Files          13       13           
  Lines         707      707           
  Branches       98       98           
=======================================
  Hits          581      581           
  Misses         91       91           
  Partials       35       35           

Flag	Coverage Δ
unittests	82.17% <ø> (ø)

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