def where bool_array
      Daru::Core::Query.vector_where self, bool_array
    end
For querying the data
@param bool_array [object] arel like query syntax
@return [Daru::Vector] Vector which makes the conditions true
@example
dv = Daru::Vector.new ['I', 'II', 'I', 'III', 'I', 'II'],
type: :category,
ordered: true,
categories: ['I', 'II', 'III']
dv.where(dv.mt('I') & dv.lt('III'))
# => #<Daru::Vector(2)>
#   1  II
#   5  II