Increase coverage

[brosaplanella]

Various fixes to increase coverage and improve docstrings.

[codecov[bot]]

Codecov Report

Base: 99.38% // Head: 99.63% // Increases project coverage by +0.25% :tada:

Coverage data is based on head (577f325) compared to base (4a8a221). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2223      +/-   ##
===========================================
+ Coverage    99.38%   99.63%   +0.25%     
===========================================
  Files          359      361       +2     
  Lines        19821    20014     +193     
===========================================
+ Hits         19699    19941     +242     
+ Misses         122       73      -49     

Impacted Files	Coverage Δ
...bamm/expression_tree/operations/evaluate_python.py	99.30% <ø> (+1.00%)	:arrow_up:
pybamm/models/event.py	100.00% <ø> (+4.16%)	:arrow_up:
pybamm/solvers/scikits_ode_solver.py	97.56% <ø> (+8.27%)	:arrow_up:
pybamm/expression_tree/concatenations.py	100.00% <100.00%> (+1.29%)	:arrow_up:
pybamm/expression_tree/symbol.py	100.00% <100.00%> (+1.20%)	:arrow_up:
...dels/full_battery_models/lead_acid/higher_order.py	98.14% <0.00%> (-0.02%)	:arrow_down:
pybamm/util.py	100.00% <0.00%> (ø)
pybamm/__init__.py	100.00% <0.00%> (ø)
pybamm/solvers/solution.py	100.00% <0.00%> (ø)
pybamm/solvers/base_solver.py	100.00% <0.00%> (ø)
... and 45 more

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[brosaplanella]

There are still some additional changes I want to do before merging

[brosaplanella]

I don't think I will have time to work on this in the next few weeks so I think it is better to merge. @tinosulzer, can you give it a quick look?