getIndeces: Get the indeces for the individual intervals
Description
This function returns the indeces of the vector coming from DFT of time series
of length n.ahead that correspond to frequencies in the interval (up, down].
Usage
getIndeces(n.ahead, up, down)
Arguments
n.ahead
the length of the vector coming out of the DFT
up
the upper boundary of the interval
down
the lower boundary of the interval