def recast opts={}
      opts.each do |vector_name, dtype|
        self[vector_name].cast(dtype: dtype)
      end
    end
Change dtypes of vectors by supplying a hash of :vector_name => :new_dtype

== Usage
df = Daru::DataFrame.new({a: [1,2,3], b: [1,2,3], c: [1,2,3]})
df.recast a: :nmatrix, c: :nmatrix