Plotting improvements

[LiamPattinson]

Upgraded TokamakSource plotting functions and expanded tests

• Can now pass Axes into plotting functions, can be used when the user wishes to make use of subplots etc.
• Operations converted to make better use of NumPy
• Better error reporting from plotting functions
• Plotting tests greatly expanded, now produces example plots. This currently creates about 25 pngs in the tests folder, which may not be appropriate behaviour.

[codecov[bot]]

Codecov Report

Merging #55 (af1fa5e) into develop (6944d70) will increase coverage by 0.26%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #55      +/-   ##
===========================================
+ Coverage    96.07%   96.34%   +0.26%     
===========================================
  Files            7        7              
  Lines          204      219      +15     
===========================================
+ Hits           196      211      +15     
  Misses           8        8              

Impacted Files	Coverage Δ
...enmc_plasma_source/plotting/plot_tokamak_source.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 6944d70...af1fa5e. Read the comment docs.