phantom: correct ellipses for the original Shepp-Logan model.
* phantom.m: change the additive intensity of the first ellipse from 2 to 1
so it is displayed correctly in doube precision (even though original paper
does use a value of 2. See comments on source for details). Correct additive
intensity of ellipse "g" from 0.02 to 0.01. Rewrite documentation to use
the deftypefn macros. Remove unecessary subfunctions select_phantom() and
read_args() which are only called once anyway.
* NEWS: mention changes to function.