save result

[SarahRo]

Description

Add save and save_data functions to pybop.result.

Fixes #866

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.97%. Comparing base (4628a65) to head (44e9a36).
⚠️ Report is 17 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #873      +/-   ##
===========================================
+ Coverage    89.41%   90.97%   +1.56%     
===========================================
  Files           63       67       +4     
  Lines         4857     5343     +486     
===========================================
+ Hits          4343     4861     +518     
+ Misses         514      482      -32     

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