isFreight attribute only assigned to freight agents

The population generated has some pecularities that MATSim users and those working with the .xml files may want to know.

• the attribute "isFreight" is unqiue to freight agents. So if you use this attribute as a condition, you may get null pointers. For example, if (person.getAttributes().getAttribute("isFreight").toString.equals("false")) will give you a null point exception. Instead, do something like this: if (person.getAttributes().getAttribute("isFreight") == null)