powspace Function

  function powspace(start,stop,num,base) result(array)
    real(8)          :: start,stop,step,array(num)
    integer          :: num,i
    real(8),optional :: base
    real(8)          :: base_
    if(num<0)stop "powspace: N<0, abort."
    base_= 2.d0;if(present(base))base_=base
    array(1) = start
    forall(i=2:num)array(i)=start + (stop-start)*base_**(-num+i)
  end function powspace