Fix indexing bug with infeasible experiments for IDAKLUSolver

[MarcBerliner]

Description

@pipliggins figured out why infeasible experiments were failing with the IDAKLUSolver:

I checked this with #4534 to confirm it's not an output_variables specific problem - it's not, but related.

Having the voltage limit violation at the start of a cycle step results in the final solution containing an empty array in the middle of all_ys. The assumption leading to the output_variables error was that the only time an empty numpy array would be present in all_ys is if the solver isn’t returning the full state vector, as with IDAKLU(output_variables=true), so an false error is being flagged. I fixed that by checking for an empty t array too in _update_variables(), i.e.:

      for i, (model, ts, ys, inputs, var_pybamm) in enumerate(
          zip(self.all_models, self.all_ts, self.all_ys, self.all_inputs, vars_pybamm)
      ):
          if (
              ys.size == 0
              and var_pybamm.has_symbol_of_classes(
                  pybamm.expression_tree.state_vector.StateVector
              )
              and not ts.size == 0
          ):

But using this I get a different error related to the hermite interpolation. Turning off hermite interpolation in the solver (and with the output_variables fix) the example runs as expected. I suspect the interpolation issue is also being caused by an unexpected empty array in the state vector, but haven’t dug that far.

Originally posted by @pipliggins in #4496

I added additional checking in the ProcessedVariable class to ignore empty sub-solutions.

Fixes #4496

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.

• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (164f71e) to head (b68b139).
Report is 198 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4541      +/-   ##
===========================================
- Coverage    99.42%   99.42%   -0.01%     
===========================================
  Files          299      299              
  Lines        22717    22716       -1     
===========================================
- Hits         22586    22585       -1     
  Misses         131      131              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[martinjrobins]

Thanks @MarcBerliner and @pipliggins. just one comment that will hopefully simplify the logic

[martinjrobins]

Looks good 👍