; docformat =  'rst'
;+
;Use a transformation structure to transform a vector or grid of vectors.
;
; :Examples:
;   transform = transformgrid(trans_struct, vectors)
;-
;+
; :Params:
;   T : in, required, type=array of double
;     Either a 3x3 array representing a transformation matrix, or a 4 
;     element vector representing a quaternion.
;   V : 
;     Either a 3 element vector, or an Nx3 element array where each row
;     represents a vector, or a ...x3 element grid, where each grid cell
;     represents a vector as used in resolve_grid.
;     
; :Returns:
;   A vector or grid of vectors representing the original input 
;   transformed by the transformation structure.
;-
function TransformGrid,T,V
  if(n_elements(T) eq 9) then begin
    ;It's a matrix
    return,V ## T
  end else begin
    ;It's a quaternion
    return,quat_vect_mult(T,V);
  end
end