small changes to the unit test for class LinearEvaporationPolicy
made EvaporationPolicy a standard class over pure virtual interface. The class now provides an implementation for the checkForEvaporation() method since the 'logic' of the function is the same for every derived EvaporationPolicy (this also goes for the determineEvaporationFactor)
added method for determining the time difference between current time and the last access time
fixed evaporation method of class CubicEvaporationPolicy
changed unit test for class EvaporationPolicy
addd test for class CubicEvaporationPolicy