fix for when only a subset of input params are used for sensitivities

[martinjrobins]

Fixes #1887

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: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three 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

[codecov]

Codecov Report

Merging #1897 (b8e097e) into develop (278b9f7) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1897      +/-   ##
===========================================
- Coverage    99.35%   99.35%   -0.01%     
===========================================
  Files          346      346              
  Lines        18999    18972      -27     
===========================================
- Hits         18876    18849      -27     
  Misses         123      123              

Impacted Files	Coverage Δ
pybamm/experiments/experiment.py	100.00% <ø> (ø)
...models/full_battery_models/lead_acid/basic_full.py	100.00% <ø> (ø)
...ybamm/models/full_battery_models/lead_acid/loqs.py	100.00% <ø> (ø)
...odels/full_battery_models/lithium_ion/basic_dfn.py	100.00% <ø> (ø)
...odels/full_battery_models/lithium_ion/basic_spm.py	100.00% <ø> (ø)
...tery_models/lithium_ion/electrode_soh_half_cell.py	100.00% <ø> (ø)
pybamm/discretisations/discretisation.py	99.79% <100.00%> (ø)
...bamm/expression_tree/operations/replace_symbols.py	100.00% <100.00%> (ø)
pybamm/models/base_model.py	99.23% <100.00%> (-0.02%)	⬇️
...m/models/full_battery_models/base_battery_model.py	99.76% <100.00%> (-0.01%)	⬇️
... and 18 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 804ea24...b8e097e. Read the comment docs.

[valentinsulzer]

Looks good, see suggestion for additional test