class TF1Convolution

 (f*g)(t) = int(f(x)g(x-t)dx)   *

 class wrapping convolution of two function : evaluation of TF1(t) * TF1(x-t)

 The convolution is performed by default using FFTW if it is available .
 One can pass optionally the range of the convolution (by default the first function range is used).
 Note that when using Discrete Fouriere Transform (as FFTW), it is a circular transform, so the functions should be
 approximatly zero at the end of the range. If they are significantly different than zero on one side (e.g. the left side)
 a spill over will occur on the other side (e.g right side).
 If no function range is given by default the function1 range + 10% is used
 One shoud use also a not too small number of points for the DFT (a minimum of 1000).  By default 10000 points are used .