write output to nested directory, rather than root directory

instead of filling up D??/... with lots of imputed and rdata files, save
them down at the pointy end of the stick.

utility function get.save.path from full path to file
modified:   lib/utils.R

use get.save.path
modified:   lib/load.pems.raw.file.R
modified:   lib/vds_impute.R