dropLink function

dropLink A function to drop a person from his/her parents in the simulated pedigree data.frame. The person can be dropped by specifying his/her ID or by specifying the generation which the randomly to-be-dropped person is in. The function can separate one pedigree into two pedigrees. Separating into small pieces should be done by running the function multiple times. This is a supplementary function for simulatePedigree.