Plot two properties as a scatter plot

[TomSlater]

The Particle_list.plot() function is now able to plot a scatter plot of two properties if two properties are passed as a list.

I have also added the ability to plot one or two properties from multiple Particle_lists on the same plot by adding a plot function to the api.

[codecov[bot]]

Codecov Report

Merging #25 into master will increase coverage by 2.36%. The diff coverage is 84.44%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   59.09%   61.45%   +2.36%     
==========================================
  Files          13       14       +1     
  Lines         726      768      +42     
==========================================
+ Hits          429      472      +43     
+ Misses        297      296       -1

Impacted Files	Coverage Δ
ParticleSpy/tests/test_plotting.py	100% <100%> (ø)
ParticleSpy/api.py	85.71% <75%> (-14.29%)	:arrow_down:
ParticleSpy/ptcl_class.py	79.27% <83.33%> (+9.27%)	:arrow_up:

[TomSlater]

This should implement #23