added function to remove vertex neighbour (but not connected vertex neighbour)

Samo Penic

2010-11-28 9802f169c159d66159bc15fce86c4aaa3c9e9ce5

1
Original written by Dan Kroll in FORTRAN. Code was then largely adapted and commented by Miha Fosnaric-Fosni. Rewritten into C by Samo Penic-Peni, with help of Miha's extensive explanations.