Set a default aggregation for growthandeatlengths and preylengths

[bthe]

When defining a stock length aggregations are required for both the growth and the consumption process. This is done to speed up calculations when the aggregations are defined on coarser scale than the model. Defining these files however is typically a nuisance as the most common scenarios the are the same files and they correspond to the length groups defined in the header. I suggest that the definition of these should be made optional.