Issue 3298 heaviside shape

[rtimms]

Description

Fixes shape error with Heaviside

Fixes #3298

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 run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

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

Patch coverage: 100.00% and no project coverage change.

Comparison is base (840fb13) 99.70% compared to head (460e540) 99.70%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3306   +/-   ##
========================================
  Coverage    99.70%   99.70%           
========================================
  Files          248      248           
  Lines        18890    18894    +4     
========================================
+ Hits         18835    18839    +4     
  Misses          55       55           

Files Changed	Coverage Δ
pybamm/expression_tree/binary_operators.py	100.00% <100.00%> (ø)

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[brosaplanella]

Looks good, thanks!