Move grav2 sample data to subdirectory, so I can add other data

[skip ci]
Revise example for read_cross2, b/c grav2 data now in subdirectory

[skip ci]
Add ReadMe files for scripts and sample data [skip ci]
Add main ReadMe for sampledata directory [skip ci]
ReadMe for inst/scripts: files in fixed-width font [skip ci]
Initial stab at a second set of sample data
[skip ci]
remove sample data; put on web, instead
read_cross2: Add link to sample data page on web
read_cross2: handle X chr, sex, and cross_info; add simplistic test
Remove RcppEigen from "Imports" (just in "LinkingTo") [skip ci]