Issues with turbulence modeling in isothermal VOF solvers

Under the VOF framework, the flow of the isothermal mixture belongs to the variable-density incompressible flow category. For such flows, VOF-based solvers of OpenFOAM fail to construct the correct governing equations for turbulence modeling. varRhoTurbVOF contains a set of newly designed VOF-based solvers which could use the desired governing equations for turbulence quantities.

Details could be found at https://doi.org/10.1016/j.cpc.2019.106876 and https://arxiv.org/abs/1811.12580v2 Implementations for different OpenFOAM versions are provided at https://github.com/wenyuan-fan/varRhoTurbVOF

## Reattaching the author to the issue ticket: @wenyuan ##