Define LogPrior, LogPDF, LogPosterior and update set_target

[NicolaCourtier]

Description

Adds separate LogPrior, LogPDF and LogPosterior classes to re-enable the comparison via plotting in the maximum_a_posteriori.ipynb example and to clarify that samplers are only appropriate for LogPDF-type problems. Also updates the set_target attribute of costs and problems.

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 95.16129% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.84%. Comparing base (4628a65) to head (28a1479).
⚠️ Report is 33 commits behind head on develop.

Files with missing lines	Patch %	Lines
pybop/problems/meta_problem.py	66.66%	3 Missing ⚠️
pybop/costs/base_cost.py	95.00%	2 Missing ⚠️
pybop/problems/log_pdf.py	93.75%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #897      +/-   ##
===========================================
+ Coverage    89.41%   92.84%   +3.42%     
===========================================
  Files           63       73      +10     
  Lines         4857     5674     +817     
===========================================
+ Hits          4343     5268     +925     
+ Misses         514      406     -108     

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