synched version of gpu convolution works
started to twist the CPU api to use an fftw_malloc/_free based allocator (check-in broken\!)
refactored cpu_convolve, cpu convolution tests all pass
single iteration tests with new cpu api work as expected
* tweaked the build system to pick up external FFTW installations,
** the way it is done seems to be recommended by the cmake people but I don't like because it clutters compile calls (shared libs are given as absolute paths etc)
** this needs to be fixed at some point

* the performance of the cpu version is not very reasonable so far, each iteration takes 400s currently (using 12 threads on a 12 CPU system), according to benchmarks with fftw, only 115s of those are due to fftw, there must be a bottleneck somewhere

* gpu deconvolution tests fails currently, because I have not yet implemented the "all-views-on-device" code path