Issue 746 broadcasting

[valentinsulzer]

Description

Remove outers and primary broadcasts from some models and do it automatically instead.

Fixes #746

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

[rtimms]

looks great thanks @tinosulzer ! much more user friendly now (once tests are fixed!)

[codecov]

Codecov Report

Merging #747 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #747      +/-   ##
==========================================
+ Coverage   98.35%   98.36%   +<.01%     
==========================================
  Files         175      175              
  Lines        9599     9880     +281     
==========================================
+ Hits         9441     9718     +277     
- Misses        158      162       +4

Impacted Files	Coverage Δ
...models/particle/fickian/fickian_single_particle.py	100% <ø> (ø)	⬆️
pybamm/models/submodels/thermal/base_thermal.py	92.5% <ø> (ø)	⬆️
pybamm/spatial_methods/spatial_method.py	95.12% <100%> (+0.06%)	⬆️
pybamm/expression_tree/binary_operators.py	98.87% <100%> (+0.02%)	⬆️
...amm/models/submodels/convection/base_convection.py	100% <100%> (ø)	⬆️
...odels/submodels/convection/composite_convection.py	100% <100%> (ø)	⬆️
.../models/submodels/convection/leading_convection.py	100% <100%> (ø)	⬆️
...ll/diffusion/composite_stefan_maxwell_diffusion.py	96.96% <100%> (-0.09%)	⬇️
...m/full_surface_form_stefan_maxwell_conductivity.py	100% <100%> (ø)	⬆️
...onductivity/leading_stefan_maxwell_conductivity.py	100% <100%> (ø)	⬆️
... and 13 more

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