explicitly allow for choice of landmark extensions
remove redundant has_landmarks, map presence used
correct definition of PointGraph classes
removed unused warnings import
Merge pull request #504 from jabooth/iolmchoice

explicitly have kwarg in IO for landmark extensions
styling fixes
Merge pull request #505 from nontas/point_directed_graph

PointGraph (PointUndirectedGraph, PointDirectedGraph, PointTree)
adds has_cycles for is_tree
minor correction
test update
Merge pull request #506 from nontas/is_tree

is_tree update in Graph