paired Difference
The paired differences as an array for the pair of data names given by the strings. If the data names don't exist then an error occurs. The paired difference is assumed to be named "$s1 - $s2$". If the named difference is not found an empty array is returned.
Return
an array of the paired differences
Parameters
s1
the name of data set number 1
s2
the name of data set number 2