LIQUID_FLOW: Replace darcy_gravity by specific_body_force

Description

Currently, the LIQUID_FLOW process takes the gravity into account via setting a positive scalar. Thereby it is assumed that gravity acts in the opposite direction of the one of the coordinate axes. In line with the other processes the definition of a signed body force should be preferred instead.

Current implementation:

<darcy_gravity>
   <!-- axis_id: 0, 1, 2 -->
   <axis_id> 0 </axis_id>
   <!-- g>=0. g=0: no gravity -->
   <g>9.81</g> <!--m/s²-->
</darcy_gravity>

Desired implementation:

<specific_body_force>0 -9.81</specific_body_force>

However, many examples then need to be adapted (automatically). Maybe it's worth it.

Edited Mar 02, 2021 by Dmitry Yu. Naumov