Port the existing graphs implementation from: https://github.com/yinonavraham/ynn-util-collections

[yinonavraham]

There is already a basic implementation of graphs under the yinonavraham/ynn-util-collections github repo. Copy it here, adjust its package name to: org.yatech.common.collection.graph, and make it more java8 style (was written on java6...).