#403 add special case for constant function parameter

[valentinsulzer]

Description

Avoids trying to process children if a function parameter is an object that doesn't depend on its children

Fixes #2704

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: $ 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

Base: 99.72% // Head: 99.72% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (7bcfd80) compared to base (8478e9a).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2477   +/-   ##
========================================
  Coverage    99.72%   99.72%           
========================================
  Files          257      257           
  Lines        18986    18988    +2     
========================================
+ Hits         18933    18935    +2     
  Misses          53       53           

Impacted Files	Coverage Δ
pybamm/parameters/parameter_values.py	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[Scottmar93]

Looks good!