WIP: Refactor dnsFoam

Summary

- If applied: This commit will restructure dnsFoam in line with other
              solvers, e.g. pisoFoam, without changing its external
              behaviour.

- Why: Prior to this change, dnsFoam structure reflected v.2.x and older
       solver style.

- How: This change collects naked dnsFoam code parts under general file
       structure, e.g. via UEqn.H.

Resolved bugs (If applicable)

N/A

Details of new models (If applicable)

N/A

Risks

- Verification: No change in any output of dnsFoam tutorial case.