EnablingAsserts

A common first "gotcha" in simulation scripts, the following macro must be declared to enable NS3_ASSERT() assertion macros:

  #define NS3_ASSERT_ENABLE

This should happen before core-module.h is included. Note that macros may be defined from the command line in gcc, e.g.:

g++ -Wall -g -DNS3_ASSERT_ENABLE -o foo src/foo.cc -lns3

This is useful to enable/disable NS3_ASSERT() without having to edit any code.