Use traits for (un)directed types

[filchristou]

I find the definition of MetaUndirectedGraph and MetaDiGraph very constrained.
Graphs.jl solves this problem by using traits.
Here, I made the changes so that the same solution is followed.

SimpleTraits.jl is already in the Manifest as an indirect dependency due to Graphs.jl

With this solution we can now build MetaGraphs of any subtype of AbstractGraph implementing Graphs.is_directed()
Before only MetaGraphs of SimpleGraph and SimpleDiGraph could be built.

Use case

For example, if this PR, together with QuantumBFS/Multigraphs.jl#18 and QuantumBFS/Multigraphs.jl#17 are completed, we will be able to have a MetaGraph of Multigraph.
This will be a huge advantage over MetaGraphs.jl, which for now it's also only limited to SimpleGraph and SimpleDiGraph.

More discussion

I don't quite agree with the Graphs.SimpleGraph and Graphs.SimpleDiGraph methods.
To me they look like constructors and they should return a new object, and not the graph of the MetaGraph.
For this reason, we could instead have something simple like a getgraph method.

[bramtayl]

Thanks! Using traits for this makes sense to me!

[bramtayl]

I also agree with you about the Graphs.SimpleGraph and Graphs.SimpleDiGraph methods being confusing. We inherited this from the original MetaGraphs

[bramtayl]

If you want to help maintain I could add you as a collaborator?

[filchristou]

Do you have any comments on the PR ? Keep in mind that this is a breaking change, since I removed MetaDiGraph and MetaUndirectedGraph. No particular reason why I removed those and one could add them back, but they go opposite to the idea of MetaGraphNext being a general wrapper.

I also agree with you about the Graphs.SimpleGraph and Graphs.SimpleDiGraph met

I made an issue if more conversation needs to happen #42
This PR already took the liberty to remove these. Let me know if that's fine.

If you want to help maintain I could add you as a collaborator?

Yes sure! I cannot promise working consistently but I will be of help, so please go on!

[bramtayl]

What do you think @gdalle?

[codecov]

Codecov Report

Merging #40 (a1ef56f) into master (f1175c1) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
- Coverage   90.90%   90.83%   -0.08%     
==========================================
  Files           8        7       -1     
  Lines         242      240       -2     
==========================================
- Hits          220      218       -2     
  Misses         22       22              

Impacted Files	Coverage Δ
src/MetaGraphsNext.jl	100.00% <ø> (ø)
src/metagraph.jl	84.61% <ø> (ø)
src/directedness.jl	100.00% <100.00%> (ø)
src/graphs.jl	83.49% <100.00%> (+0.66%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[gdalle]

Thanks for the great PR @filchristou, it's a really awesome idea!