Refect (stick) floats that hit the surface (bottom)

[arango]

A new C-preprocessing option FLOAT_STICKY was added to reflect floats that hit the surface and stick floats that hit the bottom. This is needed to avoid overshoots and spurious unmixing near the boundary when vertical random walk (FLOAT_VWALK) is activated.

Alternate options may be needed in the floats with biological behavior.

Many thanks to Mark Hadfield for contributing this option.