Remove harmonic mean regularization

[MarcBerliner]

Description

Removes a regularization factor of 1e-16 in the diffusion coefficient harmonic mean. I believe this term was included when pybamm still used normalized variables, but now it's unnecessary and causes noticeable error.

Minor things:

• Changes harmonic mean functional form to reduce the number of diffusion variables in codegen
• Reduces the size of a magic number in FunctionParameter from 1e-16 to 1e-300

Fixes #4976

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: nox -s pre-commit
• All tests pass: nox -s tests
• The documentation builds: nox -s doctests
• 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 98.64%. Comparing base (1afcea2) to head (4d76a52).
Report is 111 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4977   +/-   ##
========================================
  Coverage    98.64%   98.64%           
========================================
  Files          304      304           
  Lines        23750    23750           
========================================
  Hits         23429    23429           
  Misses         321      321           

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[aabills]

looks great, thanks!