Create a numpy adaptor for primesieve

[pstoll]

Today primesieve generates a python list of primes. We then take this list and immediately turn it into a numpy array. That is wasteful (generating the the list) and slow. We can do better - create the output numpy array and pass it to the primesieve generate_primes function, so that it writes the primes directly into the output array. This will avoid creating a huge list and copying it later.