This GUI tool wraps the Vector.correl function
http://www.neuron.yale.edu/neuron/docs/help/neuron/general/classes/vector/vect2.html#$Since Vector.correl is implemented using the fast fourier transform
this tool interpolates the vectors in the domain tbegin to tend onto
a grid with number of points equal to a power of 2. To do this an
"actual dt" is selected which is near the "nominal dt" grid spacing and
satisfies (tend-tbegin)/"actual dt" = 2^n where n is an integer.