Node-centered vs. Cell-centered data interpretation

[jalevine]

The current cleaver code (lib/FloatField.cpp) has built into it a notion of data centering. This can affect the quality of geometry coming out, since it would place the data on either the primal or dual grid, and interpolation otherwise.

This was a source of confusion at my Kitware visit. It would be handy to expose the data centering as a command line parameter (specifically the variable m_centering).