R CMD Check igraph warning

[Namita Gupta]

* checking whether package ‘alakazam’ can be installed ... WARNING
Found the following significant warnings:
  Warning: replacing previous import by ‘igraph::union’ when loading ‘alakazam’
  Warning: replacing previous import by ‘igraph::as_data_frame’ when loading ‘alakazam’
  Warning: replacing previous import by ‘igraph::groups’ when loading ‘alakazam’
  Warning: replacing previous import by ‘igraph::%>%’ when loading ‘alakazam’

[Jason Vander Heiden]

Resolved in 419d926 by using @importFrom directives to load only the bare necessities from igraph.