Add functional test dft

first commit to make dirty code cleaning on (i)dft calls (legacy walls never cleaned to use the more recent API)

second commit introduce a all dft sizes test against Gnu scientific library implementation (with double)
it tests a simple square signal in only real values

for now, the result looks ugly, even with huge tolerance thresholds

I made very large buffers because some DFT calls make several DFT in // but it is difficult to understand, for example DFT864 is either bugged or it makes 3 DFTs in one call

The largest DFT anyway makes segv