- optionally returns centre point as well

- useful at the moment for reporting face areas from a surface mesh

eg, dictionary entry:

        thermalModel
        {
            type        none; // fixedTemperature;
            // fixedTemperature coefficients
            T           350;
        }

A missing thermalModel is treated as 'none'

- we don't support paraview < 3.x anymore

- renamed clearEnv -> unset