HiC.scala
    Changed the signature InteractionGraph.addInteraction(x: Interaction, nreads: Int)
    to InteractionGraph.addInteraction(x: Interaction).  It was
    redundent to have nreads as a parameter since the information is in
    the Interaction object.

    Wrote toString methods for Interaction and InteractionGraph

HiCTests.scala
    Wrote test for InteractionGraph.toString.  This also indirectly
    tests addInteraction.