Revert split VTKRecorder, PartialSatClayEngine, TwoPhaseFlowEngine

Following consensus in #239 (closed) I am removing the splits. Fortunately in meantime there have been no changes in these files which would need to be carried over. I simply go back to these commits:

• use VTKRecorder.cpp version from commit 49691b6a
• revert commit f9f4cc27 to restore PartialSatClayEngine, TwoPhaseFlowEngine

mentioned in issue #239 (closed)

We can verify that the files are exactly the same as before the split with these commands:

git diff 49691b6a0 postprocessing/vtk/VTKRecorder.cpp
git diff 49691b6a0 postprocessing/vtk/VTKRecorder.hpp
git diff 153ab9354 pkg/pfv/PartialSatClayEngine.cpp
git diff 153ab9354 pkg/pfv/TwoPhaseFlowEngine.cpp

Using 49691b6a and 153ab935 as reference.

approved this merge request

Works perfectly, thank you again.

approved this merge request

merged