Improve docs and style

[gdalle]

• Add docstrings everywhere
• Clean up code style to activate CI style check
• Rework the README into a tutorial page
• Stress performance aspects

[gdalle]

This fixes #35 and #18

[codecov]

Codecov Report

Merging #38 (d956a63) into master (25b9d91) will decrease coverage by 1.00%.
The diff coverage is 91.73%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
- Coverage   95.82%   94.83%   -1.00%     
==========================================
  Files           6        9       +3     
  Lines         407      445      +38     
==========================================
+ Hits          390      422      +32     
- Misses         17       23       +6     

Impacted Files	Coverage Δ
src/SimpleWeightedGraphs.jl	100.00% <ø> (+10.52%)	⬆️
src/conversion.jl	75.00% <75.00%> (ø)
src/simpleweightededge.jl	82.75% <79.16%> (-1.46%)	⬇️
src/abstractsimpleweightedgraph.jl	88.88% <88.88%> (ø)
src/persistence.jl	97.56% <90.00%> (-2.44%)	⬇️
src/simpleweightedgraph.jl	93.39% <91.66%> (-0.49%)	⬇️
src/simpleweighteddigraph.jl	96.47% <98.07%> (+0.52%)	⬆️
src/overrides.jl	100.00% <100.00%> (+0.99%)	⬆️
src/utils.jl	100.00% <100.00%> (ø)

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[matbesancon]

LGTM!