Satisfy trait type condition

[filchristou]

is_directed(::Type{CustomGraph})::Bool should be defined to be consistent with Graphs.jl https://juliagraphs.org/Graphs.jl/dev/ecosystem/interface/

[codecov[bot]]

Codecov Report

Base: 92.46% // Head: 91.95% // Decreases project coverage by -0.51% :warning:

Coverage data is based on head (2537441) compared to base (f7553c7). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
- Coverage   92.46%   91.95%   -0.51%     
==========================================
  Files           5        6       +1     
  Lines         345      348       +3     
==========================================
+ Hits          319      320       +1     
- Misses         26       28       +2     

Impacted Files	Coverage Δ
src/di_multigraph_adjlist.jl	94.32% <100.00%> (-0.68%)	:arrow_down:
src/multigraph_adjlist.jl	93.16% <100.00%> (-0.81%)	:arrow_down:
src/Multigraphs.jl	100.00% <0.00%> (ø)

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