fftw plan: estimate, don't measure. accelerates tests substantially

The total execution time of the test suite can be substantially reduced if we use FFTW_ESTIMATE instead of FFTW_MEASURE for generating plans in AtomicConvolver::updateKernel.

FFTW_MEASURE is meant for programs where many Fourier transforms are executed with the same plan so that in the long run the speed gain from an optimized plan outweighs the time spent on getting the plan. Here, however, the plan is used one single time.

Further opportunities for accelerating the Fourier transform should be considered separately.