#1376 Fixed PEP styling around docstrings

[asinghgaba]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes #1376

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 not required
• Tests added that prove fix is effective or that feature works not required

[codecov]

Codecov Report

Merging #1383 (25cf8e4) into develop (92d0279) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1383   +/-   ##
========================================
  Coverage    98.25%   98.25%           
========================================
  Files          277      277           
  Lines        15689    15689           
========================================
  Hits         15415    15415           
  Misses         274      274           

Impacted Files	Coverage Δ
pybamm/citations.py	100.00% <ø> (ø)
pybamm/discretisations/discretisation.py	99.78% <ø> (ø)
pybamm/experiments/experiment.py	98.33% <ø> (ø)
pybamm/expression_tree/binary_operators.py	99.61% <ø> (ø)
pybamm/expression_tree/broadcasts.py	100.00% <ø> (ø)
pybamm/expression_tree/functions.py	100.00% <ø> (ø)
pybamm/expression_tree/input_parameter.py	100.00% <ø> (ø)
pybamm/expression_tree/symbol.py	98.03% <ø> (ø)
pybamm/expression_tree/unary_operators.py	100.00% <ø> (ø)
pybamm/geometry/geometry.py	100.00% <ø> (ø)
... and 23 more

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[valentinsulzer]

thanks @asinghgaba , merging