Develop GenericStressor-system

A GenericStressor-system is needed. It should apply a mortality on one or more species for a specific week and segment. The base input for the system is a list of a new struct, comparable to the injection structs for the pesticide modules. This could be called StressorEvents and has the variables startTime, endTime, startSeg, endSeg, yearlyRep, []species bool, intensity and type. Intensity sets the strength of the stressor application and type how this strength is applied. For now, there are two different ways: GeneralMortality and DriftMortality. The DriftMortality is applied during grazing. In the Initialization of the GenericStressor-system should be a flag setter, if true the Grazing-system calls a function from the GenericStressor-system to get the modified drift mortality.