rename CrateTermination to CRateTermination because the step is CRate

[aabills]

Description

I noticed today that to use a step for c-rate you call pybamm.step.CRate but to create a termination you use pybamm.step.CrateTermination. This is inconsistent, so this pr fixes it.

Fixes # (issue)

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 (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: $ python -m pytest (or $ nox -s tests)
• The documentation builds: $ python -m pytest --doctest-plus src (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ nox -s quick.

Further checks:

• Code is commented, particularly in 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 98.70%. Comparing base (7419219) to head (d956513).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4834   +/-   ##
========================================
  Coverage    98.70%   98.70%           
========================================
  Files          303      303           
  Lines        23314    23340   +26     
========================================
+ Hits         23011    23037   +26     
  Misses         303      303           

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[agriyakhetarpal]

Thanks, @aabills! Could you please add a test for the warning as the coverage has apparently dropped?