Generate a convolution function to remove the 3-position chop function
from raster scans SCULIB_3POS_CONFN
This function consists of a series of delta functions at a spacing equal to that between a -ve spike and the central spike of the 3-position chop. The height of the delta dunctions peaks at the centre of the convolution function and falls off by 1 at a time for the delta functions on either side, so that the envelope of the function is an isosceles triangle.
The convolution function must cover the scan even when the centre of the function is at one end of it. Hence, the convolution function must be twice the length of the scan.
Since the raw data are not sampled such that the chop spacing is an integer number of samples, the actual convolution function must be rebinned onto the sample mesh by sinc interpolation.