bound sto for better eSOH results

[valentinsulzer]

Description

Bound stoichiometry for OCV between 1e-10 and 1-1e-10. Together with the singularities introduced at 0 and 1, this ensures that the eSOH results are always between 0 and 1.

Fixes #1856

[brosaplanella]

Looks good, thanks!

[codecov]

Codecov Report

Merging #2095 (27f8cec) into develop (bc72c6d) will decrease coverage by 0.00%.
The diff coverage is 99.48%.

@@             Coverage Diff             @@
##           develop    #2095      +/-   ##
===========================================
- Coverage    99.38%   99.38%   -0.01%     
===========================================
  Files          348      348              
  Lines        19255    19234      -21     
===========================================
- Hits         19136    19115      -21     
  Misses         119      119              

Impacted Files	Coverage Δ
pybamm/spatial_methods/spatial_method.py	96.36% <ø> (ø)
pybamm/spatial_methods/finite_volume.py	98.15% <92.30%> (ø)
pybamm/discretisations/discretisation.py	99.79% <100.00%> (ø)
pybamm/expression_tree/binary_operators.py	99.53% <100.00%> (-0.01%)	⬇️
pybamm/expression_tree/concatenations.py	98.70% <100.00%> (-0.04%)	⬇️
pybamm/expression_tree/functions.py	100.00% <100.00%> (ø)
...mm/expression_tree/operations/convert_to_casadi.py	100.00% <100.00%> (ø)
...ybamm/expression_tree/operations/evaluate_julia.py	100.00% <100.00%> (ø)
...bamm/expression_tree/operations/evaluate_python.py	98.29% <100.00%> (ø)
pybamm/expression_tree/operations/jacobian.py	100.00% <100.00%> (ø)
... and 16 more

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