Add more documentation for GaussianMixtureReducer.

[mattbrown11]

Added a little more detail to the doc for GaussianMixtureReducer properties. Looks like my IDE also removed trailing whitespaces, hope that isn't an issue.

[codecov]

Codecov Report

Base: 94.80% // Head: 94.80% // No change to project coverage 👍

Coverage data is based on head (5eac282) compared to base (b33f56c).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #770   +/-   ##
=======================================
  Coverage   94.80%   94.80%           
=======================================
  Files         175      175           
  Lines        9456     9456           
  Branches     1884     1884           
=======================================
  Hits         8965     8965           
  Misses        350      350           
  Partials      141      141           

Flag	Coverage Δ
integration	69.38% <100.00%> (ø)
unittests	89.67% <100.00%> (ø)

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Impacted Files	Coverage Δ
stonesoup/mixturereducer/gaussianmixture.py	97.77% <100.00%> (ø)

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[sdhiscocks]

Thanks @mattbrown11