Add benchmark test for "heat transfer during the melting of ice around a horizontal, isothermal cylinder"

[agzimmerman]

Based on this paper: https://www.researchgate.net/publication/226804783_Heat_transfer_during_the_melting_of_ice_around_a_horizontal_isothermal_cylinder

[agzimmerman]

This involves:

• Refactor octadecane.py to make the buoyancy model extensible #242
• Bring back the water buoyancy model #243

[agzimmerman]

Alpay's presentation on this topic is scheduled for June 27.

[agzimmerman]

I fixed #242 and #243 a little while back, so this should be more straightforward now.

[agzimmerman]

This is important for demonstrating...

• a more interesting geometry than the unit square, including adaptive refinement of curved boundary.
• validation against an experiment.