Use distance_all_pairs instead of distance_matrix
Avoid unnecessary calculation of all distances
Removed unnecessary check
Fixed bug: a graph is complete if each pair of distinct vertices lies at distance 1 of each other
Removed comparison to True
Simplified if-else type of functions returning a boolean
Merge branch 'master' of https://github.com/math1um/objects-invariants-properties
Simplified if-else type of functions returning a boolean