Add Hessian check for 2-parameter problems

[NicolaCourtier]

Description

Add a classify_using_Hessian function that can assess the identifiability of an optimisation result and return an informative message to the user.

Issue reference

Fixes #362.

Review

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• Updated the CHANGELOG.md in reverse chronological order (newest at the top) with a concise description of the changes, including the PR number.
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Type of change

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[codecov]

Codecov Report

Attention: Patch coverage is 83.01887% with 9 lines in your changes missing coverage. Please review.

Project coverage is 98.50%. Comparing base (6a6b7dd) to head (1687e11).
Report is 1 commits behind head on develop.

Files	Patch %	Lines
pybop/_classification.py	83.01%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #363      +/-   ##
===========================================
- Coverage    98.76%   98.50%   -0.27%     
===========================================
  Files           48       49       +1     
  Lines         3153     3206      +53     
===========================================
+ Hits          3114     3158      +44     
- Misses          39       48       +9     

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