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.