Testing TokamakSource generates points correctly

[LiamPattinson]

I've added a test and refactored test_tokamak_source.py to ensure TokamakSource generates points within the expected shape. This change brings in hypothesis as an additional testing requirement, which is used to automatically generate a wide range of possible tokamaks and ensure it works for more than the single example given previously. It also includes a general refactor to avoid the repeated lines of code.

I believe this goes some way towards solving issue #34, but I'd like to go a bit further. For example, the inputs to TokamakSource aren't checked for validity in its __init__ function. It would be quite straightforward to implement these checks and associated unit tests, and I could raise this as separate issue/pull request.

[codecov]

Codecov Report

Merging #38 (edbc0b2) into develop (ca46cab) will not change coverage.
The diff coverage is 87.50%.

@@           Coverage Diff            @@
##           develop      #38   +/-   ##
========================================
  Coverage    96.34%   96.34%           
========================================
  Files            6        6           
  Lines          164      164           
========================================
  Hits           158      158           
  Misses           6        6           

Impacted Files	Coverage Δ
openmc_plasma_source/plotting/__init__.py	100.00% <ø> (ø)
openmc_plasma_source/point_source.py	86.66% <80.00%> (ø)
openmc_plasma_source/ring_source.py	88.88% <80.00%> (ø)
openmc_plasma_source/tokamak_source.py	97.87% <90.00%> (ø)
...enmc_plasma_source/plotting/plot_tokamak_source.py	100.00% <100.00%> (ø)

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[RemDelaporteMathurin]

Sorry somehow I wasn't notified of this PR thanks @shimwell for pointing it out. I shall have a look asap!

[RemDelaporteMathurin]

Hi @LiamPattinson thank you so much for this very nice refactore!

I just had a few questions and minor pep8 style to apply here and there.

Otherwise I'm very happy with this PR!

[shimwell]

I am not sure why the pep workflow called black didn't run on this

[LiamPattinson]

Apologies for missing the PEP8 formatting, still trying to get in the habit of calling black regularly. Also not sure why it was missed by the CI.