SpssReader: handle StringValues properly.
package net.tz.spss
case class SpssVariable(name: String, shortName: String, isNumeric: Boolean,
index: Int, label: String,
missingRange: Option[DoubleRange] = None,
missing: List[Double] = Nil,
missingStrings: List[String] = Nil,
labels: Map[Double, String] = Map())