Individuals are assigned to populations based on the
individual/sample/specimen metrics file (csv) used with gl.read.dart().
One might want to define the population structure in accordance with another
classification, such as using an individual metric (e.g. sex, male or
female). This script discards the current population assignments and replaces
them with new population assignments defined by a specified individual
metric.
The script returns a genlight object with the new population assignments.
Note that the original population assignments are lost.