Implment simpler layer constructors

TODO:

- write tests
- implement a version of those constructors that takes a list of nodes as `vertices` and a list of 2-tuples of `MultilayerVertex`s as `edge_list`.

Co-Authored-By: Pietro Monticone <38562595+pitmonticone@users.noreply.github.com>
Co-Authored-By: Claudio Moroni <43729990+ClaudMor@users.noreply.github.com>

Author: 3 people

docs/src/API.md

@@ -112,6 +112,7 @@ mv_outneighbors(subgraph::AbstractSubGraph, mv::MultilayerVertex)
 mv_neighbors(subgraph::AbstractSubGraph, mv::MultilayerVertex)
 has_edge(subgraph::AbstractSubGraph,me::MultilayerEdge)
 has_edge( subgraph::AbstractSubGraph, s::MultilayerVertex, d::MultilayerVertex)
+has_edge( layer::Layer, s::MultilayerVertex{nothing}, d::MultilayerVertex{nothing})
 ne(subgraph::AbstractSubGraph)
 edges(subgraph::S) where {T,U,S<:AbstractSubGraph{T,U}} 
 add_edge!( subgraph::S, me::E) where {T,U<:Real,S<:AbstractSubGraph{T,U},E<:MultilayerEdge{ <: Union{U, Nothing}}}

src/MultilayerGraphs.jl

@@ -75,6 +75,14 @@ export
     AbstractLayer,
     Layer,
     simple_layer,
+    simple_dilayer,
+    simple_weighted_layer,
+    simple_weighted_dilayer,
+    metadigraph_layer,
+    layer_valgraph,
+    layer_valoutdigraph,
+    layer_valdigraph,
+    meta_layer,
     has_node,
     add_vertex!,
     rem_vertex!,

src/graphs_extensions/metagraphs.jl

@@ -63,7 +63,7 @@ function _add_edge!(
 ) where {T<:Integer,W<:Union{<:Real,Nothing}}
     (isnothing(weight) || weight == 1.0) ||
         @warn "Trying to add a weighted edge to an unweighted graph of type $(typeof(g)). Weight $weight will be ignored."
-    return add_edge!(g, src, dst, Dict(key => value for (key, value) in pairs(metadata)))
+    return add_edge!(g, src, dst, Dict(Symbol(pair.first) => pair.second for pair in pairs(metadata) )) #Symbol(string(key)) => value for (key, value) in pairs(metadata)
 end
 
 function _get_edge_weight(