Enforce `print_citations()` in notebooks

All notebooks should have the cell pybamm.print_citations() at the end, to encourage people to use this command. This can be enforced using regex when running the example tests.

[brosaplanella]

Sounds good! It seems that in order to print all the citations you need to pass "[*]". I am happy to give it a go as it lines up with #1182 which I need to tackle at some point.

[rtimms]

pybtex seems to work ok now

e.g.

import pybamm
import pybtex

model = pybamm.lithium_ion.SPM()

sim = pybamm.Simulation(model)
sim.solve([0, 3600])
pybamm.print_citations(filename="test.bib")
print(pybtex.format_from_file("test.bib", "unsrt", output_backend="plaintext"))

you can output to markdown too and pick different styles