Add ImmutableMixIn and Freezable classes

[erogers-dstl]

For discussion: I wrote these in a separate repo for use in a internal project.

Are these something of use to the community?

[erogers-dstl]

Also: it is possible this shouldn't be in base.py. Would a new file be more appropriate?

[codecov[bot]]

Codecov Report

Base: 94.58% // Head: 94.59% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (4c9dc7c) compared to base (ed7ffb0). Patch coverage: 96.77% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #715      +/-   ##
==========================================
+ Coverage   94.58%   94.59%   +0.01%     
==========================================
  Files         172      172              
  Lines        8841     8903      +62     
  Branches     1721     1734      +13     
==========================================
+ Hits         8362     8422      +60     
- Misses        346      347       +1     
- Partials      133      134       +1     

Flag	Coverage Δ
integration	69.89% <35.48%> (-0.24%)	:arrow_down:
unittests	90.74% <96.77%> (+0.04%)	:arrow_up:

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

Impacted Files	Coverage Δ
stonesoup/base.py	95.60% <96.77%> (+0.34%)	:arrow_up:

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