Issue 774 remove outer kron

[valentinsulzer]

Description

Remove Outer and Kron nodes as they are no longer needed

Fixes #774

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 #777 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #777      +/-   ##
==========================================
+ Coverage   98.42%   98.44%   +0.01%     
==========================================
  Files         179      179              
  Lines        9931     9890      -41     
==========================================
- Hits         9775     9736      -39     
+ Misses        156      154       -2

Impacted Files	Coverage Δ
pybamm/expression_tree/operations/evaluate.py	95.65% <ø> (+0.86%)	⬆️
pybamm/discretisations/discretisation.py	99.74% <ø> (-0.01%)	⬇️
...mm/expression_tree/operations/convert_to_casadi.py	100% <100%> (ø)	⬆️
pybamm/expression_tree/binary_operators.py	99.05% <100%> (+0.18%)	⬆️
pybamm/spatial_methods/spatial_method.py	95.78% <100%> (+0.23%)	⬆️
pybamm/expression_tree/operations/jacobian.py	100% <100%> (ø)	⬆️

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[rtimms]

looks good to me, thanks