TPC: fix initialization of FFTW arrays in aggregator-ft

[ehellbar]

No description provided.

[ehellbar]

@matthias-kleiner

[ehellbar]

with a4bd6bc, --nthreads, which sets the dimension of the FT vectors, was moved to a local spec level option. Therefore, setNThreads() is now called in the init() after the FT vectors were already initialized in the constructor via initFFTW3Members().

This PR makes initFFTW3Members() a public function which resizes the vectors according to sNThreads. It is now called in init() after setNThreads().

NB: whenever using this class standalone, one needs to call initFFTW3Members() explicitly after constructing the object in order allocate the fftw vectors (before it was done in the constructor).

[matthias-kleiner]

Thanks for fixing it!