WIP: polymake posets

[benlorenz]

Extracted from #3819, but might need some things from there to work.

Todo:

• tests
• examples
• do we need PartiallyOrderSetElement ? (create from constructor?)
• can we properly visualize the top node?
• make sure bottom and top are first and last node (for order polytope & co)?
• what kind of input do we need? (currently: full graph, graph with rank map, matrix of covering relations, incidencematrix for set inclusions (like face lattices))

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 76 lines in your changes missing coverage. Please review.

Project coverage is 83.88%. Comparing base (686a746) to head (fb7200c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3928      +/-   ##
==========================================
- Coverage   83.99%   83.88%   -0.11%     
==========================================
  Files         591      592       +1     
  Lines       81386    81462      +76     
==========================================
- Hits        68359    68336      -23     
- Misses      13027    13126      +99     

Files	Coverage Δ
src/Oscar.jl	68.35% <ø> (ø)
src/Combinatorics/PartiallyOrderedSet/functions.jl	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes