Shifts a kinship matrix
kinshipShift(x,shift=NULL)a square matrix
the amount by which the elements of x should be shifted. if shift==NULL,
the average of the off-diagonal elements is substracted
the shifted kinship matrix \(\frac{x-shift}{1-shift}\)
The kinship matrix produced by beta.dosage is relative to the average kinship
of the set of individuals analysed (\(1/(n(n-1)/2) \sum_i \sum_{j>i} x_{ij}=0\)).
Another reference point might be useful, for instance to avoid negative kinship values, one might
want to shift the matrix by \(min(x_{ij}), i \neq j\).