added EdgeHandler for handling map edges
added EdgeHandledPotentialMap with getter for edge handler
ConvolvingPotentialMap.java
    implements EdgeHandledPotentialMap and returns the one from its convolveOp
FlowFromPotentialsMap.java
    handles edges according to map's edge handler
        default to extend instead of returning zero potentials
    added test for custom edge handler
PotentialMap.java
    added java 8 TODO
SimplePotentialMap.java
    implements EdgeHandledPotentialMap
    added field edgeHandler and accessor with value set by constructor
ConvolveOp.java
    uses newly added EdgeHandler instead of internal EdgeHint
    updated test
Double2DCloseTo.java
    added default max error and factory method using it
renamed TestConstantPathfindingMap to TestPathfindingMap
    updated TestConstantFlowMap