Create parent Result class

[NicolaCourtier]

Description

Create a parent class for all types of optimisation and sampling result to clarify which attributes are shared.

Also, move PosteriorSummary attributes to the SamplingResult and deprecate the pints.AdaptiveCovarianceMCMC sampler.

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 90.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.73%. Comparing base (4628a65) to head (7359637).
⚠️ Report is 10 commits behind head on develop.

Files with missing lines	Patch %	Lines
pybop/_result.py	88.88%	1 Missing ⚠️
pybop/plot/contour.py	50.00%	1 Missing ⚠️
pybop/plot/convergence.py	50.00%	1 Missing ⚠️
pybop/plot/parameters.py	75.00%	1 Missing ⚠️
pybop/plot/voronoi.py	50.00%	1 Missing ⚠️
pybop/samplers/mcmc_summary.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #860      +/-   ##
===========================================
+ Coverage    89.41%   90.73%   +1.31%     
===========================================
  Files           63       67       +4     
  Lines         4857     5214     +357     
===========================================
+ Hits          4343     4731     +388     
+ Misses         514      483      -31     

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[SarahRo]

Overall this looks good to me and makes the code cleaner.

There is just one minor thing: I find it somewhat confusing where in the code the subclasses of Result are defined. OptimisationResult is defined in base_optimiser.py, but SamplingResult in mcmc_summary. Would it maybe make sense to define them in their own file?