[bugfix] Base.eltype works on graph types as well as instances

[lmondada]

According to the Julia documentation, eltype should be defined on the ::Type{AbstractGraph} instead of the instances themselves:

"The definition eltype(x) = eltype(typeof(x)) is provided for convenience so that instances can be passed instead of types. However the form that accepts a type argument should be defined for new types."

Currently, however:

julia> eltype(SimpleGraph{Int64})
Any

julia> eltype(SimpleGraph{Int64}())
Int64

This PR fixes this to return

julia> eltype(SimpleGraph{Int64})
Int64

[etiennedeg]

It makes me wonder if it's okay to call this method eltype since AbstractGraph is not iterable. I don't think it is too bad, we should better not break anything.

[gdalle]

I removed the implementations for Simple(Di)Graph and moved the fallback to AbstractGraph. As a result, eltype does not need to be part of the interface anymore

[codecov]

Codecov Report

Merging #144 (90afd95) into master (5649092) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   97.23%   97.26%   +0.02%     
==========================================
  Files         114      114              
  Lines        6583     6579       -4     
==========================================
- Hits         6401     6399       -2     
+ Misses        182      180       -2