added MapContent as nested class and renamed it to Changes
        made immutable
        added width and height fields and package-private accessors
        added accessors for collections
        constructor is package-private
            can be created from nested Factory class or content method
extracted super class AbstractDerivedMap from DerivedFlowMap
    added method content
    changed content constructor not containing firstMap anymore
        width/height now derived from content object
    clear no longer returns a collection
        unneccessary because of content method
    fixed always setting weight by adding weight parameter to addMapInternal
DerivedFlowMap.java
    fixed inspector title
FlowFromPotentialsMap, FlowFromFlowsMap.java
    updated constructor with changes object
    added constructor with weight for first map