smart calculation of nfft in Trace.remove_response

in case large primes are encountered in nfft factorization (which makes
the computation take forever) we simply switch to using the next power
of 2, which is not the fastest possibility but has robust computation
time.
nfft: try to avoid going up to power of 2, try out some slightly larger
values before that
nfft: avoid problems with large primes in seisSim, too

changelog
nfft calculation: more tries to find good nfft number, computation time
of factorization is negligible compared to fft/evalresp/ifft

(e.g. for a trace with 1e7 points: factorization is <1ms but response
calculation is >1min)
fix previous commits
Merge remote-tracking branch 'megies/smart_nfft' into releases (see #715)