read_cross2: additional args in call to data.table

 - attempt to suppress messages
 - data.table=FALSE makes the output a plain data.frame
c++ code: classes RIsib, RIself -> RISIB, RISELF
c++: return(blah); -> return blah; (in a few cases, for consistency)
Add DH and haploid cross types; redundant, but what the hell
Remove RcppEigen from NAMESPACE as not needed
Add check_crosstype(), for checking if a cross type is supported
Re removing RcppEigen from NAMESPACE: I'd forgotten to re-run make to update the NAMESPACE file
Add convert2cross2() for converting R/qtl cross to R/qtl2 format