Need print methods for S3 objects that are intermediate outputs from qtl2 and qtl2pleio

[fboehm]

in JOSS review, Vince Carey suggested this. Think about how to summarize (for printing purposes) the intermediate objects that I create in the vignettes.

openjournals/joss-reviews#1435

[fboehm]

I might contact Karl (author of rqtl/qtl2) to ask whether print methods are desired for qtl2 outputs.