Addtion of basic Digital Signal Processing algorithms

[NoeMurr]

Following the discussion #67 I propose the introduction of few basic DSP algorithms.
Taking inspiration from the armadillo C++ library I would like to propose the introduction of this 4 algorithms to start:

1. conv
2. fft / ifft
3. conv2
4. fft2 / ifft2

For the implementation of the fft algorithm I propose to include a "simple" version to use when performance are not so important and to give the opportunity to the user to configure the library to use an external library like rustFFT when performance are a key aspect.