Issue 749 cache symbol shape

[valentinsulzer]

Description

Save symbol shapes when they have already been evaluated

Fixes #749

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

❗ No coverage uploaded for pull request base (master@d05306b). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #780   +/-   ##
=========================================
  Coverage          ?   98.43%           
=========================================
  Files             ?      179           
  Lines             ?     9937           
  Branches          ?        0           
=========================================
  Hits              ?     9781           
  Misses            ?      156           
  Partials          ?        0

Impacted Files	Coverage Δ
pybamm/models/standard_variables.py	100% <ø> (ø)
pybamm/expression_tree/binary_operators.py	98.87% <100%> (ø)
pybamm/expression_tree/concatenations.py	97.95% <100%> (ø)
pybamm/models/submodels/particle/base_particle.py	97.29% <100%> (ø)
pybamm/expression_tree/unary_operators.py	96.14% <100%> (ø)
pybamm/expression_tree/broadcasts.py	100% <100%> (ø)
pybamm/expression_tree/operations/jacobian.py	100% <100%> (ø)
pybamm/spatial_methods/spatial_method.py	95.55% <100%> (ø)
pybamm/expression_tree/functions.py	100% <100%> (ø)
pybamm/spatial_methods/finite_volume.py	97.46% <100%> (ø)
... and 7 more

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