cyclone-physics: Buoyancy force appears to be calculated incorrectly

In my opinion the calculation of the buoyancy is incorrect (file pfgen.cpp lines 120 and 121).

I think the correct calculation should look like this:

    force.y = liquidDensity * volume *
        (depth - maxDepth - waterHeight) / (2 * maxDepth);

Current code is missing the parentheses around “2 * maxDepth”