initial values instead of mean

[SarahRo]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context.

Fixes # (issue)

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 69.23077% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.36%. Comparing base (e63f2e9) to head (a8665e1).
⚠️ Report is 7 commits behind head on add-plot-predictive.

Files with missing lines	Patch %	Lines
pybop/optimisers/ep_bolfi_optimiser.py	75.00%	5 Missing ⚠️
pybop/costs/feature_distances.py	60.00%	2 Missing ⚠️
pybop/costs/base_cost.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##           add-plot-predictive     #919      +/-   ##
=======================================================
- Coverage                92.48%   92.36%   -0.13%     
=======================================================
  Files                       74       74              
  Lines                     5724     5725       +1     
=======================================================
- Hits                      5294     5288       -6     
- Misses                     430      437       +7     

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[NicolaCourtier]

Let's merge these commits and keep working on PR #918