FR: make unique = TRUE default for CJ

[jaapwalhout]

The unique-parameter of CJ was initially set to FALSE when introduced. The relevant item in the newsfile when 1.9.6 was released said:

CJ gains logical unique argument with default FALSE. If TRUE, unique values of vectors are automatically computed and used. This is convenient, for example, DT[CJ(a, b, c, unique=TRUE)] instead of doing DT[CJ(unique(a), unique(b), unique(c))]. Ultimately, unique = TRUE will be default. Closes #1148.

As CJ is probably mostly used used with unique = TRUE (see also the original issue), maybe the time has come to set this parameter to unique = TRUE ?

[jangorecki]

Anyone depending on duplicates can be badly affected. We could make 1.12.2-1.13.0 a transition period where extra check for uniqueness could be added. In case of detecting duplicates and missing(unique) a warning could be raise about changing that default from 1.13.0.