update scripts to future proof against R 4.0.0+ updates

• https://stat.ethz.ch/pipermail/r-announce/2020/000653.html

  R now uses a stringsAsFactors = FALSE default, and hence by default no longer converts strings to factors in calls to data.frame() and read.table().

• this breaks all instances of levels() being used on data.frames read using read.csv()

• switching to unique() solves this

• unique() used on factors (R < 4.0.0, currently on the cluster) still returns the same values as levels(), but as a factor, which does not cause problems (but is a little ugly in interactive use)