Implement Shaffer's multiple comparison procedure

See https://www.jstor.org/stable/2289016, see also https://projecteuclid.org/eBooks/institute-of-mathematical-statistics-lecture-notes-monograph-series/Recent-Developments-in-Multiple-Comparison-Procedures/Chapter/Implementing-Shaffers-multiple-comparison-procedure-for-a-large-number-of/10.1214/lnms/1196285622

These procedures improve upon Holm's method used by default in adastop.

