Convience method for dealing with factors. Map a list of vectors to a list of factor vectors (1-1 mapping) such that
the factor vectors all have the same levels - the unique values of the union of all the vectors in the list. Optionally
group all low frequency values into a "_other_" level.
Usage
set_factor(vectorList, aggregationThreshold = 0)
Arguments
vectorList
A list of values to convert to factors
aggregationThreshold
Values which appear this many times or less will be grouped into the level "_other_"