Implement weights for LMM in scan1blup

[kbroman]

Implement the use of weights in scan1() and related functions (scan1coef(), fit1(), scan1blup(), scan1perm(), scan1snps()). This is for the case of RIL (such as the BXDs) where we're analyzing strain means with differing numbers of individuals per line.

The variance part of the model will be v[ h^2 K + (1-h^2) D] where D is a diagonal matrix with the reciprocal of the weights, 1/w, along the diagonal.

[kbroman]

I have the basics of this working in scan1() in the weighted_lmm branch. Need to add some tests (verified with the regress package).

Also need to implement the weights argument for LMM in

• fit1()
• scan1coef()
• scan1snps()
• scan1blup()
• scan1perm()