Bug fixes, output in Magnitude

Bug fixes:
- The functions to add to and pop from a tossll
  had a bug: they only changed one of the largest
  or smallest pointers, that is why in some conditions
  the next and previous pointers would go crazy! But now
  both work on both pointers if necessary and the problem
  is fixed.

- When the center of a profile is near the side of a
  pixel, and the profile is highly elliptical, then that
  pixel is not necessarily the one that is closest to the
  profile center. I added fsp_checkell() to mock.c to check
  if this happens. It can be further optimzed later to
  analytically find the correct pixel. I will do that
  in the future.

Last column of output is now in terms of magnitude,
not simple flux. In order to do that I had to add an option
of zero point magnitude to MockGals' options. I changed the
corresponding sections in the manual.