Create 4-way cross type

[kbroman]

Create a cross type that's just like "4way" in R/qtl1 (phase-known 4-way cross), but with sex-averaged genetic map, since we've not implemented sex-specific maps in R/qtl2.

Could switch to using separate offspring and founder genotypes, but maybe best to just copy over exactly what we did in R/qtl1, with possible marker genotypes 1, 2, ..., 14, as a complicated coding.

My biggest concern is that the genotypes AC,AD,BC,BD, will not work right with various existing code, which generally assumes either all homozygotes, or all combinations of homozygotes and heterozygotes.