Remove .id

[valentinsulzer]

Description

Adds __eq__ and __hash__ methods for pybamm.Symbol (possible now we're not using anytree)
Replaces symbol.id with symbol in dictionary keys and for equality checking, where possible

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 #1978 (f1954ef) into develop (e1f52ff) will decrease coverage by 0.00%.
The diff coverage is 99.46%.

@@             Coverage Diff             @@
##           develop    #1978      +/-   ##
===========================================
- Coverage    99.38%   99.38%   -0.01%     
===========================================
  Files          348      348              
  Lines        19255    19228      -27     
===========================================
- Hits         19136    19109      -27     
  Misses         119      119              

Impacted Files	Coverage Δ
pybamm/spatial_methods/spatial_method.py	96.36% <ø> (ø)
pybamm/spatial_methods/finite_volume.py	98.15% <92.30%> (ø)
pybamm/discretisations/discretisation.py	99.79% <100.00%> (ø)
pybamm/expression_tree/binary_operators.py	99.53% <100.00%> (-0.01%)	⬇️
pybamm/expression_tree/concatenations.py	98.70% <100.00%> (-0.04%)	⬇️
pybamm/expression_tree/functions.py	100.00% <100.00%> (ø)
...mm/expression_tree/operations/convert_to_casadi.py	100.00% <100.00%> (ø)
...ybamm/expression_tree/operations/evaluate_julia.py	100.00% <100.00%> (ø)
...bamm/expression_tree/operations/evaluate_python.py	98.29% <100.00%> (ø)
pybamm/expression_tree/operations/jacobian.py	100.00% <100.00%> (ø)
... and 14 more

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[rtimms]

thanks @tinosulzer !

[brosaplanella]

Looks good, just a couple of small comments. Thanks Valentin!

[brosaplanella]

I am a bit confused, why is the .id here now?

[valentinsulzer]

we still need id here to generate names like cache_12345. I guess it could go into the id_to_julia_variable function

[brosaplanella]

Ah, I get it. Looks good then!

[brosaplanella]

Same here