add empty inits to parameters subfolders

[TomTranter]

Description

When running scripts through spyder on windows (at least this is what I tested) if you press run twice there is an importlib KeyError. The kernel had to be restarted to fix the issue. Adding empty __init__files to parameter subfolders fixes the issue

Fixes #1479

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

Merging #1555 (4095246) into develop (f83f0a2) will decrease coverage by 0.50%.
The diff coverage is n/a.

❗ Current head 4095246 differs from pull request most recent head 68a8f8c. Consider uploading reports for the commit 68a8f8c to get more accurate results

@@             Coverage Diff             @@
##           develop    #1555      +/-   ##
===========================================
- Coverage    98.28%   97.78%   -0.51%     
===========================================
  Files          310      323      +13     
  Lines        17402    17491      +89     
===========================================
  Hits         17103    17103              
- Misses         299      388      +89     

Impacted Files	Coverage Δ
...ectrolyte_exchange_current_density_Ramadass2004.py	0.00% <0.00%> (ø)
...o2_Ramadass2004/lico2_entropic_change_Moura2016.py	0.00% <0.00%> (ø)
...ectrodes/lico2_Ai2020/lico2_ocp_Ai2020_function.py	0.00% <0.00%> (ø)
...des/nmc_Chen2020/nmc_LGM50_diffusivity_Chen2020.py	0.00% <0.00%> (ø)
...rodes/lico2_Ramadass2004/lico2_ocp_Ramadass2004.py	0.00% <0.00%> (ø)
...Ramadass2004/graphite_entropic_change_Moura2016.py	0.00% <0.00%> (ø)
...ectrolyte_exchange_current_density_Ramadass2004.py	0.00% <0.00%> (ø)
...electrodes/nca_Kim2011/nca_ocp_Kim2011_function.py	0.00% <0.00%> (ø)
...2004/graphite_mcmb2528_diffusivity_Dualfoil1998.py	0.00% <0.00%> (ø)
...graphite_Ramadass2004/graphite_ocp_Ramadass2004.py	0.00% <0.00%> (ø)
... and 3 more

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[rtimms]

I guess we also need these for lead acid

[TomTranter]

Yep looks like you do. Just tried running compare_extrapolations twice and it failed. This is confusing slightly as I would have thought it would just break on import pybamm

[TomTranter]

@rtimms @tlestang I am not sure how to test this on codecov - can you merge without?

[rtimms]

thanks @TomTranter ! Yeah don't worry about coverage