#1070 add length scales

[rtimms]

Description

Add length scales to effective resistance models. Also fix effective volumetric heat capacity, which was missing the current collector contributions.

Fixes #1070

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 #1071 into develop will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1071   +/-   ##
========================================
  Coverage    97.72%   97.72%           
========================================
  Files          237      237           
  Lines        12435    12437    +2     
========================================
+ Hits         12152    12154    +2     
  Misses         283      283           

Impacted Files	Coverage Δ
...ollector/effective_resistance_current_collector.py	96.39% <100.00%> (+0.03%)	⬆️
pybamm/parameters/standard_parameters_lead_acid.py	100.00% <100.00%> (ø)
...bamm/parameters/standard_parameters_lithium_ion.py	100.00% <100.00%> (ø)

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