Divided by 3600 to replace t0_cr

[DrSOKane]

Description

The old non-dimensionalized PyBaMM had a constant t0_cr used for the crack propagation submodel. This was deleted, so I divided dl_cr/dt by 3600 (standard value for t0_cr) to compensate and retreive the same results as existing PyBaMM.

Fixes #2418

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all
• The documentation builds: $ python run-tests.py --doctest

You can run unit and doctests together at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

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[valentinsulzer]

Great! Thanks very much

[codecov]

Codecov Report

Base: 99.68% // Head: 99.68% // No change to project coverage 👍

Coverage data is based on head (339b745) compared to base (73b2a3c).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@                   Coverage Diff                   @@
##           issue-2418-dimensional    #2724   +/-   ##
=======================================================
  Coverage                   99.68%   99.68%           
=======================================================
  Files                         270      270           
  Lines                       18945    18945           
=======================================================
  Hits                        18886    18886           
  Misses                         59       59           

Impacted Files	Coverage Δ
.../submodels/particle_mechanics/crack_propagation.py	100.00% <100.00%> (ø)

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