'Set the clip\'s position in compositions.
Sets the position that the clip will have when included
in compositions. The argument ``pos`` can be either a couple
``(x,y)`` or a function ``t-> (x,y)``. `x` and `y` mark the
location of the top left corner of the clip, and can be
of several types.
Examples
>>> clip.set_pos((45,150)) # x=45, y=150
>>> # clip horizontally centered, at the top of the picture
>>> clip.set_pos(("center","top"))
>>> # clip is at 40% of the width, 70% of the height:
>>> clip.set_pos((0.4,0.7), relative=True)
>>> # clip\'s position is horizontally centered, and moving up !
>>> clip.set_pos(lambda t: (\'center\', 50+t) )'

@apply_to_mask
@outplace
def set_position(self, pos, relative=False):

    self.relative_pos = relative
    if hasattr(pos, '__call__'):
        self.pos = pos
    else:
        self.pos = (lambda t: pos)
