Add erosion to watershed process

[TomSlater]

Add erosion step to watershed process. This erodes the binary image before a distance map is calculated.

• [x] Add code

• [x] Add documentation

• [x] Add tests

[codecov[bot]]

Codecov Report

Merging #47 into master will increase coverage by 8.07%. The diff coverage is 86.36%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
+ Coverage   72.97%   81.04%   +8.07%     
==========================================
  Files          14       13       -1     
  Lines         792      781      -11     
==========================================
+ Hits          578      633      +55     
+ Misses        214      148      -66     

Impacted Files	Coverage Δ
ParticleSpy/segptcls.py	66.66% <66.66%> (-1.69%)	:arrow_down:
ParticleSpy/ParticleAnalysis.py	94.33% <100.00%> (+19.46%)	:arrow_up:
ParticleSpy/tests/test_particle_analysis.py	100.00% <100.00%> (ø)
ParticleSpy/tests/test_particle_clustering.py	100.00% <100.00%> (ø)
ParticleSpy/particle_save.py	20.00% <0.00%> (-3.08%)	:arrow_down:
ParticleSpy/particle_load.py	100.00% <0.00%> (ø)
ParticleSpy/particle_io.py