scan1boot: bootstrap scan1()

[kbroman]

There was a message on the R/qtl2 google group about bootstrapping scan1(), analogous to scanoneboot() in R/qtl1.

Here's an initial stab at it: scan1boot.R gist.

Maybe take a chromosome argument and focus on just one chromosome and have the output be just a matrix of positions? Or have an option of the full output vs the matrix.