added interface GridFilteringOp.java
added AbstractGridFilteringOp.java
    extracted basic workings from ConvolveOp
renamed ConvolvingPotentialMap to FilteringPotentialMap.java
    updated to accept less specific interface GridFilteringOp
        instead of ConvolveOp
    renamed test
ConvolveOp.java
    extends from AbstractFilteringOp
Kernel.java, de.zmt.pathfinding.filter.package-info.java
    improved doc