#1986 remove zero electrolyte concentration cut-off

[valentinsulzer]

Description

Removes the "Zero electrolyte concentration cut-off" event.
Adds some bounds to functions so that the solver still works even if we step into electrolyte depletion

Fixes #1986

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 #2207 (36870f3) into develop (bb15440) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #2207   +/-   ##
========================================
  Coverage    99.36%   99.36%           
========================================
  Files          355      355           
  Lines        19486    19499   +13     
========================================
+ Hits         19362    19375   +13     
  Misses         124      124           

Impacted Files	Coverage Δ
...models/full_battery_models/lead_acid/basic_full.py	100.00% <ø> (ø)
...odels/full_battery_models/lithium_ion/basic_dfn.py	100.00% <ø> (ø)
..._battery_models/lithium_ion/basic_dfn_half_cell.py	99.01% <ø> (ø)
...lyte_conductivity/base_electrolyte_conductivity.py	98.68% <ø> (ø)
...dels/electrolyte_conductivity/full_conductivity.py	100.00% <ø> (ø)
...lectrolyte_diffusion/base_electrolyte_diffusion.py	100.00% <ø> (ø)
...ybamm/models/submodels/interface/base_interface.py	100.00% <ø> (ø)
...electrolyte_conductivity/composite_conductivity.py	100.00% <100.00%> (ø)
...lectrolyte_conductivity/integrated_conductivity.py	100.00% <100.00%> (ø)
...e_potential_form/full_surface_form_conductivity.py	100.00% <100.00%> (ø)
... and 4 more

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[brosaplanella]

Looks good, thanks!