Enforce print_citations() in notebooks

[brosaplanella]

Description

• Notation in citations has been homogenised.
• pybamm.print_citations() has been extended to print formatted citations (both in screen and to files).
• Added examples on how to cite in Tutorials 1 and 2 (the comment in Tutorial 1 is a bit longer as it is the introduction).
• Test to check if notebooks include pybamm.print_citations() (commented out for the moment).

Fixes #1330

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

[valentinsulzer]

Looks great, thanks!

[codecov]

Codecov Report

Merging #1340 (ac929cb) into develop (810ee79) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1340   +/-   ##
========================================
  Coverage    98.14%   98.14%           
========================================
  Files          272      272           
  Lines        15515    15528   +13     
========================================
+ Hits         15227    15240   +13     
  Misses         288      288           

Impacted Files	Coverage Δ
pybamm/parameters/parameter_sets.py	100.00% <ø> (ø)
pybamm/citations.py	100.00% <100.00%> (ø)
...ybamm/models/full_battery_models/lead_acid/full.py	100.00% <100.00%> (ø)
...dels/full_battery_models/lead_acid/higher_order.py	98.09% <100.00%> (ø)
...ybamm/models/full_battery_models/lead_acid/loqs.py	100.00% <100.00%> (ø)
...odels/full_battery_models/lithium_ion/basic_dfn.py	100.00% <100.00%> (ø)
..._battery_models/lithium_ion/basic_dfn_half_cell.py	99.31% <100.00%> (ø)
...odels/full_battery_models/lithium_ion/basic_spm.py	100.00% <100.00%> (ø)
...bamm/models/full_battery_models/lithium_ion/dfn.py	100.00% <100.00%> (ø)
...bamm/models/full_battery_models/lithium_ion/spm.py	100.00% <100.00%> (ø)
... and 19 more

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