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Rename force to forceDensity in LBM force models

Christoph Schwarzmeier requested to merge rename-force-forcedensity into master

This merge request renames "force" to "forceDensity" in the LBM force models. With this change, we aim to emphasize that the user must specify a body force density in waLBerla's force models.

The force is defined as \boldsymbol{F} = m \rho \boldsymbol{a}, where m = \Delta x^{3} \rho is a cell's mass, \rho is the macroscopic fluid density, and \boldsymbol{a} is an acceleration. The force density is \boldsymbol{F_\rho} = \boldsymbol{F} / \Delta x^{3} = \rho \boldsymbol{a}, because we use \Delta x = 1 in waLBerla.

When using a incompressible lattice model, the density does not change and is equal to the LBM reference density \rho_{0}=1. Therefore, \boldsymbol{F} = \boldsymbol{F_\rho}. In contrast, when using a compressible lattice model, the density may vary in space and time, so that \boldsymbol{F} \neq \boldsymbol{F_\rho}.

Additionally, this merge request adds a warning when Guo's forcing model is used in conjunction with a TRT or MRT collision operator. This warning addresses #176 and should be removed once the issue is resolved in !560.

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