Add `igraph_is_complete()` to R

[maelle]

h/t @szhorvat

[clpippel]

My preference is_complete() as in:

is_acyclic                 
is_bipartite               is_chordal       is_connected                    
is_dag                     is_degseq        is_directed                     
is_forest                  is_graphical     is_hierarchical                 
is_igraph                  is_isomorphic_to is_matching                     
is_max_matching            is_min_separator is_named                        
is_printer_callback        is_separator     is_simple                       
is_subgraph_isomorphic_to  is_tree          is_weighted

[szhorvat]

Agreed with @clpippel on the naming.

@maelle What is "h/t"?

[maelle]

@szhorvat heard through

[clpippel]

@szhorvat, I Heard It Through the Grapevine ?

I do not want to be pedantic, however I saw it as hat tip (h/t), or tip of the hat.