This function allows to define a global attribute of the nodeset (i.e a variable that is identical for each node but changes over time).
defineGlobalAttribute(global)
an object of class global.goldfish
For instance, seasonal climate changes could be defined as a
changing global attribute.
Then, this global attribute can be linked to the nodeset by using
linkEvents()
seasons <- defineGlobalAttribute(data.frame(time = 1:12, replace = 1:12))