#1399 fix polynomial and average bugs

[rtimms]

Description

Fixes two (unrelated) bugs where:

1. volume averaging with 0D spatial method gave the incorrect result
2. polynomial particle concentration models with "dimensionality" equal to 2 you gave incorrect behaviour

Fixes #1396 and #1399

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

[valentinsulzer]

Thanks @rtimms , looks good to me!

[codecov]

Codecov Report

Merging #1411 (e16928a) into develop (44398b9) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1411      +/-   ##
===========================================
+ Coverage    98.22%   98.27%   +0.04%     
===========================================
  Files          278      278              
  Lines        15804    16110     +306     
===========================================
+ Hits         15524    15832     +308     
+ Misses         280      278       -2     

Impacted Files	Coverage Δ
...m/models/full_battery_models/base_battery_model.py	99.68% <ø> (+<0.01%)	⬆️
pybamm/models/standard_variables.py	100.00% <ø> (ø)
...dels/submodels/interface/kinetics/base_kinetics.py	100.00% <ø> (ø)
pybamm/experiments/experiment.py	98.34% <100.00%> (+0.01%)	⬆️
pybamm/expression_tree/unary_operators.py	100.00% <100.00%> (ø)
pybamm/models/base_model.py	99.20% <100.00%> (+0.09%)	⬆️
...s/submodels/particle/polynomial_single_particle.py	100.00% <100.00%> (ø)
pybamm/plotting/quick_plot.py	100.00% <100.00%> (ø)
pybamm/simulation.py	97.94% <100.00%> (+0.74%)	⬆️
pybamm/solvers/base_solver.py	99.15% <100.00%> (+0.03%)	⬆️
... and 35 more

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