Convert documentation to Google-style strings and give it a full look over

[gcassella]

I've used docconvert (https://github.com/cbillingham/docconvert) to autoconvert our docstrings from reStructuredText to Google style docstrings. There are a few other things we should do before merging the PR:

• Look over all the docstrings and tidy them up. Try and conform to google style: the first line of the docstring should be a single line (you can have additional paragraphs below) and explain briefly what the function returns.
• Add module level docstrings to the top of files (explaining what kind of functions can be found in that module / should be placed there)
• Make sure the docstrings all actually match the function signatures

If we want to go full Google, here is our bible: https://google.github.io/styleguide/pyguide.html

[codecov-commenter]

Codecov Report

Merging #21 (22d4c00) into main (cb52dcc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #21   +/-   ##
=======================================
  Coverage   76.28%   76.28%           
=======================================
  Files          15       15           
  Lines        1324     1324           
=======================================
  Hits         1010     1010           
  Misses        314      314           

Impacted Files	Coverage Δ
src/koala/chern_number.py	100.00% <ø> (ø)
src/koala/example_graphs.py	64.95% <ø> (ø)
src/koala/flux_finder/flux_finder.py	82.60% <ø> (ø)
src/koala/flux_finder/pathfinding.py	92.98% <ø> (ø)
src/koala/graph_color.py	89.39% <ø> (ø)
src/koala/graph_utils.py	48.27% <ø> (ø)
src/koala/hamiltonian.py	100.00% <ø> (ø)
src/koala/lattice.py	98.44% <ø> (ø)
src/koala/phase_diagrams.py	0.00% <ø> (ø)
src/koala/phase_space.py	95.55% <ø> (ø)
... and 3 more

---

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[gcassella]

I might also take this opportunity to snipe any linting errors and add pylint to our CI pipeline

[gcassella]

for future reference, these changes can be summarized by the commands:

pip install docconvert yapf pylint
docconvert --in-place --input rest --output google --threads 4 ./src/koala/
wget https://raw.githubusercontent.com/Imperial-CMTH/koala/documentation_station/pylintrc
yapf -i --style google ./src/koala/
pylint ./src/koala/