BRIEF DESCRIPTION OF THE DRAWINGS [0029] Embodiments of apparatus according to the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [0030]FIG. 1 is a schematic of the main elements of a first embodiment of apparatus according to the invention from the side; [0031]FIG. 2 is a schematic of the apparatus of FIG. 1 from above; [0032]FIG. 3