add function for livestock biomass consumption

[fdschneider]

Write a function that extracts from a given landscape object or list of landscapes the current amount of annual productivity in terms of livestock consumption.

This should sum up the plants' probability of dying due to grazing in the upcoming timestep. (excluding intrinsic mortality rates)

[fdschneider]

done in commit 0a3178b

its output is integrated in summary.landscape_list() and can be accessed by

summary(L100)$feed

I also added a description and details into the package vignette: inst/doc/socialecological.pdf