check buchheim assumtions

[hexaeder]

graph has to be "rooted tree"

• node 1 is root
• unique pathes (only 1 parent per node)
• all nodes can be reached

fixes #9

[codecov]

Codecov Report

Merging #34 (7d5afe3) into master (63c5fcf) will decrease coverage by 0.57%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
- Coverage   98.02%   97.44%   -0.58%     
==========================================
  Files           8        8              
  Lines         455      470      +15     
==========================================
+ Hits          446      458      +12     
- Misses          9       12       +3     

Impacted Files	Coverage Δ
src/buchheim.jl	97.19% <100.00%> (-1.60%)	⬇️
src/sfdp.jl	98.30% <0.00%> (+0.02%)	⬆️
src/NetworkLayout.jl	94.44% <0.00%> (+0.15%)	⬆️

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[hexaeder]

Hey @SimonDanisch, what do you think? I totally forgot about that one :D
This might be slow for very big graphs. This might be also breaking for some cases (there are cases where the old Buchheim gives results even though some of the assumptions are broken.
Not having those assumptions might lead to infinite loops or strange errors. I'd say merge and tag minor 🤷‍♂️