Add Multiple Model Framework

[jswright-dstl]

This PR adds the components for the Multiple model Framework and provides a tutorial introducing the KF, GPB1, GPB2 and IMM implemented through the Multiple Model Framework.

[codecov]

Codecov Report

❌ Patch coverage is 91.94313% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.45%. Comparing base (8f9cfb3) to head (812e78d).

Files with missing lines	Patch %	Lines
stonesoup/reducer/base.py	70.45%	11 Missing and 2 partials ⚠️
stonesoup/reducer/model.py	88.23%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1257      +/-   ##
==========================================
+ Coverage   91.92%   94.45%   +2.53%     
==========================================
  Files         233      245      +12     
  Lines       16843    17053     +210     
  Branches     2372     2397      +25     
==========================================
+ Hits        15483    16108     +625     
+ Misses       1006      636     -370     
+ Partials      354      309      -45     

Flag	Coverage Δ
integration	66.92% <89.09%> (?)
unittests	91.84% <89.57%> (-0.06%)	⬇️

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