Issue 632 average second dimension

[valentinsulzer]

Description

Add functionality to allow averaging in a secondary dimension (for example, averaging particle concentrations in the x-dimension in the DFN)

Fixes #632

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 #1057 into develop will increase coverage by 0.08%.
The diff coverage is 99.21%.

@@             Coverage Diff             @@
##           develop    #1057      +/-   ##
===========================================
+ Coverage    97.63%   97.72%   +0.08%     
===========================================
  Files          237      237              
  Lines        12386    12426      +40     
===========================================
+ Hits         12093    12143      +50     
+ Misses         293      283      -10     

Impacted Files	Coverage Δ
pybamm/expression_tree/binary_operators.py	94.67% <87.50%> (-0.21%)	⬇️
pybamm/discretisations/discretisation.py	99.77% <100.00%> (+<0.01%)	⬆️
pybamm/expression_tree/broadcasts.py	100.00% <100.00%> (ø)
pybamm/expression_tree/concatenations.py	97.45% <100.00%> (ø)
pybamm/expression_tree/functions.py	100.00% <100.00%> (ø)
pybamm/expression_tree/independent_variable.py	100.00% <100.00%> (ø)
pybamm/expression_tree/input_parameter.py	100.00% <100.00%> (ø)
pybamm/expression_tree/operations/simplify.py	95.14% <100.00%> (ø)
pybamm/expression_tree/symbol.py	97.44% <100.00%> (-0.02%)	⬇️
pybamm/expression_tree/unary_operators.py	100.00% <100.00%> (+2.90%)	⬆️
... and 11 more

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[brosaplanella]

Looks great Tino, thanks! Just the comment on uncommenting some of the models in compare_lead_acid.py.

[brosaplanella]

I think these should not be commented.