[iebaltab] - testpairs option

[kbjarkefur]

To allow for full customization of the table add option testpairs() that takes values on the format 1_2 where the two number in each pair corresponds to values used in the grpvar() variable, and the pairs 1_2, 1_3 etc. lists the pairwise tests to be included.

Allows for example testpairs(3_1 3_2 4_5) or whatever user wants.