Added a function to plot "summary variables"

[Saransh-cpp]

Description

Added a function to plot and compare summary variables.

Fixes # (issue)

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 #1678 (d077706) into develop (261f6c4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1678   +/-   ##
========================================
  Coverage    99.27%   99.27%           
========================================
  Files          342      343    +1     
  Lines        18720    18742   +22     
========================================
+ Hits         18585    18607   +22     
  Misses         135      135           

Impacted Files	Coverage Δ
pybamm/__init__.py	94.91% <100.00%> (+0.04%)	⬆️
pybamm/plotting/plot_summary_variables.py	100.00% <100.00%> (ø)

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[brosaplanella]

Looks good, thanks Saransh!

[valentinsulzer]

Thanks, looks good!

[valentinsulzer]

can this be made to work if only a single solution is provided not in a list, e.g.

if isinstance(solutions, pybamm.Solution):
    solutions = [solutions]