Add function to grab a subset of phenotype columns

 - e.g., to perform genome scan on just a subset of phenotypes
Add additional command-line arguments to scanone

 - phencol: phenotypes columns to use for genome scan
 - sex: name of phenotype corresponding to sex
 - crossdir: name of phenotype corresponding to cross direction
Slight improvements in foreach loops in create_phenotype_batches
Another slight improvement in a foreach loop