Doc and tests overhaul

[gdalle]

This PR does not change anything about the package functionality, but it improves overall code quality by:

• Adding quality checks with Aqua.jl to the test suite (fixed an undefined export as a result)
• Adding a formatting check with JuliaFormatter.jl to the test suite (based on the BlueStyle) $\implies$ changes in the source files are only format-related
• Moving the actual package tests from doctests to .jl files so that we can distinguish between them (doctests only give one success/fail output and do not give any more internal insight)
• Re-integrating the package tests into the documentation with Literate.jl
• Adding a little more detail to the tutorials
• Adding docstrings to the API reference for functions that are extended from Base and Graphs.jl

[codecov]

Codecov Report

Merging #43 (72c57dc) into master (5044a95) will decrease coverage by 2.97%.
The diff coverage is 94.33%.

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
- Coverage   94.28%   91.32%   -2.97%     
==========================================
  Files           7        8       +1     
  Lines         245      242       -3     
==========================================
- Hits          231      221      -10     
- Misses         14       21       +7     

Impacted Files	Coverage Δ
src/MetaGraphsNext.jl	100.00% <ø> (ø)
src/graphs.jl	82.82% <88.00%> (-6.07%)	⬇️
src/dict_utils.jl	94.28% <100.00%> (-2.86%)	⬇️
src/metadigraph.jl	100.00% <100.00%> (ø)
src/metagraph.jl	84.61% <100.00%> (-2.89%)	⬇️
src/metaundigraph.jl	100.00% <100.00%> (ø)
src/persistence.jl	100.00% <100.00%> (ø)
src/weights.jl	100.00% <100.00%> (ø)
... and 3 more

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[bramtayl]

Sure, this looks great!