Update the samplers to align saving of result

[NicolaCourtier]

Description

Move the plotting out of the Sampler class. Add result.load and update result.load_data to recreate a Result object.

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #).

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: $ pre-commit run or $ nox -s pre-commit (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: nox -s tests
• The documentation builds: nox -s doctest
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

❌ Patch coverage is 98.55072% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.63%. Comparing base (a43d72c) to head (b6a3e98).
⚠️ Report is 1 commits behind head on save-result.

Files with missing lines	Patch %	Lines
pybop/plot/samples.py	97.61%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           save-result     #896      +/-   ##
===============================================
- Coverage        92.67%   92.63%   -0.05%     
===============================================
  Files               71       72       +1     
  Lines             5639     5619      -20     
===============================================
- Hits              5226     5205      -21     
- Misses             413      414       +1     

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