ENH: Improve usability of the submarine phantom
ENH: Improve usability of the submarine phantom

- Add to __all__
- Make it use a taper parameter with default 20 instead of hard-coded
  constant c
- Scale the phantom according to the domain size
- Simplify some internal code and use better variable names

kohr-h/odl,aringh/odl,odlgroup/odl,aringh/odl,kohr-h/odl,odlgroup/odl