Fix a bunch of small problems

[kbroman]

• Make the use of vdiffr in tests conditional on it being installed
• Fix Issue calc_genoprob should give better error re missing genetic map #157, to have calc_genoprob() give better errors re missing genetic map
• Fix Issue Have read_cross2() give warning not error when possible #178, to have read_cross2() give a warning rather than error re no. alleles
• Fix Issue If the phenotypes' rownames themselves have names... #180 re scan1() giving error if phenotype rownames have names
• .Rbuildignore: avoid .RData files in subdirectories