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.

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[codecov]

Codecov Report

Attention: Patch coverage is 96.96970% with 2 lines in your changes missing coverage. Please review.

Project coverage is 99.19%. Comparing base (c8b00e6) to head (bac1082).
Report is 43 commits behind head on develop.

Files with missing lines	Patch %	Lines
pybop/_classification.py	96.92%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #363      +/-   ##
===========================================
+ Coverage    99.08%   99.19%   +0.10%     
===========================================
  Files           52       59       +7     
  Lines         3605     4333     +728     
===========================================
+ Hits          3572     4298     +726     
- Misses          33       35       +2     

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[NicolaCourtier]

I think we should continue with the slightly low coverage (98%) - the Hessian check is missing a test on the case where the optimal point is a saddle point for which we would need a relevant optimisation problem (this relates to #391).

[BradyPlanden]

Thanks @NicolaCourtier, a few suggestions/comments

[BradyPlanden]

LGTM - just one suggestion.