Add caching of mean and covar on ParticleState

[sdhiscocks]

This has significant performance improvements, for the trade off of limiting editing to state vector and weight of the ParticleState. However, they can still be replaced with another array, clearing the cache.

Minor issue is use of standard library functools.cached_property is only available from Python 3.8, so to maintain compatibility with Python 3.7, cached_property code snippet is also included.

[codecov[bot]]

Codecov Report

Merging #686 (e6b3b49) into main (bfbe6ec) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #686      +/-   ##
==========================================
+ Coverage   94.60%   94.63%   +0.02%     
==========================================
  Files         170      171       +1     
  Lines        8548     8575      +27     
  Branches     1654     1659       +5     
==========================================
+ Hits         8087     8115      +28     
+ Misses        342      339       -3     
- Partials      119      121       +2     

Flag	Coverage Δ
integration	68.52% <81.81%> (+<0.01%)	:arrow_up:
unittests	92.15% <100.00%> (+0.03%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
stonesoup/types/_util.py	100.00% <100.00%> (ø)
stonesoup/types/state.py	99.39% <100.00%> (-0.61%)	:arrow_down:
stonesoup/plotter.py	96.03% <0.00%> (+1.48%)	:arrow_up:

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