Improve memory use in experiments

[brosaplanella]

Description

Improve memory use in experiments by redefining what steps are unique. Two steps are considered the same if type, value, termination and temperature are the same, as these are the variables used when processing the model. The remaining variables are only used when running the model and can be passed as inputs. I have also defined a basic_repr which returns a representation with only those variables, while __repr__ returns a representation with all the variables.

Fixes #3081

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: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s 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

Patch coverage: 100.00% and no project coverage change.

Comparison is base (6622ec6) 99.55% compared to head (cdc6d83) 99.55%.
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3261   +/-   ##
========================================
  Coverage    99.55%   99.55%           
========================================
  Files          253      253           
  Lines        19553    19558    +5     
========================================
+ Hits         19466    19471    +5     
  Misses          87       87           

Files Changed	Coverage Δ
pybamm/solvers/solution.py	100.00% <ø> (ø)
pybamm/experiment/experiment.py	100.00% <100.00%> (ø)
pybamm/simulation.py	100.00% <100.00%> (ø)
pybamm/step/_steps_util.py	100.00% <100.00%> (ø)

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[rtimms]

thanks @brosaplanella looks good to me!