nl[,]<- should look after rownames / ordering

[jefferis]

see https://github.com/jefferis/nat/tree/feature/set-data.frame branch.

[jefferis]

[,]<- approach is obvious (same as data.frame<-) but what's the correct approach to this when doing nl[i,j] replacements. I guess we

• don't want to add rows
• might want to re-order rows if value is a data.frame (or matrix)
• in fact if value is a data.frame/matrix we probably always want to make checks