Added strong_product, disjunctive_product, lexicographical_product, h…

[dstahlke]

Added most of the graph products from https://en.wikipedia.org/wiki/Graph_product

[codecov[bot]]

Codecov Report

Merging #154 (b7fab71) into master (897e183) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   97.40%   97.43%   +0.03%     
==========================================
  Files         109      109              
  Lines        6470     6560      +90     
==========================================
+ Hits         6302     6392      +90     
  Misses        168      168              

[aurorarossi]

Hi! We recently added a JuliaFormatter and this pull request was made earlier, so it was not aligned. I fixed this problem and now the controls are fine except for one, since the homomorphic product is not tested. It would be great if you could provide one. Thank you!

[gdalle]

Making a round of unresolved PR conversations

Metadata is not supported anywhere in Graphs.jl, that is why I don't think it's a requirement for graph products.

The real issue is that these products require adding edges, and we can't guarantee this is supported beyond Simplegraphs. Furthermore, there is the question of types: what should a product of two GridGraphs be for instance? Fall back on a SimpleGraph type?

[dstahlke]

Is there anything needed from me? Should I make it take only SimpleGraph arguments rather than AbstractGraph? And if so, should I make the already existing graph product functions do the same?

[gdalle]

I think restricting these new functions to AbstractSimpleGraph would make sense for now, but without touching the existing ones (cause that might be breaking)