MultithreadCorner/Hydra: Hydra 2.3.1

CHANGE LOG Hydra 2.3.1 Interfaces to FFTW and CuFFT for performing 1D real-real complex-real and real-complex FFT on CPU and GPU FFT based convolution on CPU and GPU for arbitrary pair of functors: hydra::convolute and hydra::ConvolutionFunctor Cubic spiline reimplementation for deal with abitrary large datasets: hydra::spiline and hydra::SpilineFunctor new examples showing how to deploy convolution in fits, spilines and FFT. Many bug fixes across the tree..