Add option for different scaling methods

can you implement the Singular Value Partition methods (Yan 2002) option for GEN and ENV in the gge library? This option can be very useful for "gge" users.

Yan W. (2002). Singular-value partitioning in biplot analysis of multi-environment trial data. Agron. J. 94, 990–996.
https://dl.sciencesocieties.org/publications/aj/abstracts/94/5/990