Simplify and correct

Fixed the problem of increasing resources (they shouldnt -- misplaced
minus sign)
Simplified the main loop and adjusted where/when we calculate N from B
and apply extinction
Generate all years of tauP at the beginning -- should help with plotting
tauP when we get to that
Note that the timestep must remain small or R overshoots to <0
I added a threshold function so that R can't go below zero