Allow Fillers in P-reps design when the number of field plots ends up in a prime number.

[DidierMurilloF]

Sometimes in a p-rep design, the number of field plots is prime. In this case, it is impossible to factorize this number to offer users a set of field dimensions (Rows, Columns). One option to overcome this issue is allowing a few Filler plots that enable the user to make the randomization.
The tricky part of implementing this is the optimization process behind the p-rep design.