Issue 1220 optimizations

[valentinsulzer]

Description

Various optimizations based on running some profiling

• Cache shape and size calculations
• Rewrite is_constant
• Only create one instance of each of geometric, electrical, and thermal parameters
• Some small checks to avoid unnecessary recomputations wherever possible

Highlighted effects:

• 4x speedup on creating the DFN model object (0.4s -> 0.1s)
• The casadi solver with "fast" mode is now much faster on the second run, since it avoids calculating sizes and re-creating the integrator (around 2-3x speedup, but this depends on the grid size and parameters and model)

Fixes #1220

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 #1222 into develop will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1222   +/-   ##
========================================
  Coverage    97.92%   97.92%           
========================================
  Files          249      249           
  Lines        14146    14186   +40     
========================================
+ Hits         13852    13892   +40     
  Misses         294      294           

Impacted Files	Coverage Δ
pybamm/expression_tree/array.py	100.00% <100.00%> (ø)
pybamm/expression_tree/binary_operators.py	95.53% <100.00%> (+0.02%)	⬆️
pybamm/expression_tree/concatenations.py	97.48% <100.00%> (+0.03%)	⬆️
pybamm/expression_tree/functions.py	100.00% <100.00%> (ø)
pybamm/expression_tree/input_parameter.py	100.00% <100.00%> (ø)
pybamm/expression_tree/parameter.py	100.00% <100.00%> (ø)
pybamm/expression_tree/scalar.py	100.00% <100.00%> (ø)
pybamm/expression_tree/symbol.py	97.88% <100.00%> (+0.42%)	⬆️
pybamm/expression_tree/unary_operators.py	100.00% <100.00%> (ø)
pybamm/geometry/battery_geometry.py	100.00% <100.00%> (ø)
... and 15 more

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[brosaplanella]

Looks good, thanks Tino! Happy to merge when coverage has been fixed.