New functions for exploring multiple-QTL models

The scanqtl function, while completely flexible and so suitable for most purposes, is rather cumbersome to use. Our most important additions are the functions addqtl, to scan for a single QTL to be added to a multiple-QTL model, and addpair, to scan for a pair of QTL to be added to a multiple-QTL model. The output of these functions is of the forms produced by scanone and scantwo, respectively, and so one may use the corresponding plot and summary functions, making the results easier to deal with. For most purposes, these functions will be sufficient and direct calls to scanqtl will no longer be needed. Thus, we will not discuss the use of scanqtl further here.