def compute text, &block
      return instance_eval(&block) if block_given?
      instance_eval(text)
    end
Returns a vector, based on a string with a calculation based
on vector.

The calculation will be eval'ed, so you can put any variable
or expression valid on ruby.

For example:
a = Daru::Vector.new [1,2]
b = Daru::Vector.new [3,4]
ds = Daru::DataFrame.new({:a => a,:b => b})
ds.compute("a+b")
=> Vector [4,6]