Create a postprocessing function for cfd-dem

[yashuuzi]

Description of the problem

• There were no post-processing functions for cfd-dem, so we need framework to organize them
• We need the post-processing function which calculate total volume in order to check if total volume is conserved

Description of the solution

• Create postprocessing_cfd_dem.cc and call it in cfd_dem_coupling.cc
• Create some prameters for prm file

How Has This Been Tested?

• Launched several example, and confirmed that the volume calculated with analytical solution is matched with the volume of geometry

Documentation

• I didn't add this to the documentation or doxygen

[voferreira]

As this introduces a parameter, this parameter needs to be added to the documentation. Additionally, since this feature is used to have an assessment of the accuracy of the void fraction scheme, I feel it would be very useful to have application tests using it. This way, if we try to improve something on the current schemes, we can make sure it gives the same result.

[blaisb]

@yashuuzi can you re-indent the code?

[yashuuzi]

Thank you so much for reviewing! I'll fix them and let you know when I've done

[yashuuzi]

I'm ready for second review! Actually I could't come up with good name of parameter which is better than "volume phases", but I think it's understandable.

[voferreira]

Please, don't forget to rebase. Your branch is 20 commits behind master.

[blaisb]

@yashuuzi Is this ready for a final review / merge? Try to finish this as a priority :)

[yashuuzi]

I finished modifying and ready to final review!