def to_category *names
      names.each { |n| self[n] = self[n].to_category }
      self
    end
Converts the specified non category type vectors to category type vectors
@param [Array] names of non category type vectors to be converted
@return [Daru::DataFrame] data frame in which specified vectors have been
converted to category type
@example
df = Daru::DataFrame.new({
a: [1, 2, 3],
b: ['a', 'a', 'b']
})
df.to_category :b
df[:b].type
# => :category