Setup Testcase for cpu - gpu comparison

As discussed in the VF Meetings, we want to have a simple test case comparing the cpu and gpu part of VirtualFluids. As a quality check, we expect to get the same results accordingly to the floating-point accuracy. The test case have the following setup:

• cubic main grid
• sphere solid object
• cubic area of refinement around the sphere.

@m.schoenherr @k.kutscher Please specify the test case here:

• grid size
• MSch: 3x3x3 blocks (CPU) maybe 20x20x20 nodes per block (overall cube 60x60x60 nodes, more nodes are better ;-) )
• position / size of sphere
• MSch: in the center of the refined block
• position / size of fine grid
• MSch: the refined block/grid is located in the center of the testcase
• Which Collision Kernel?
• MSch: actual version of the K17
• Which Refinement? Probably !162 (merged) ?
• MSch: jep, the newest version of the grid refinement
• Boundary Conditions
• MSch: maybe periodic BC in all three directions (and a forcing) - check of the diff. BCs is an extra test
• MSch: we nee to evaluate flows in all three directions to be sure, that we are independent of the simulated direction
• MSch: calculation in double and single precision?

I probably miss some points. Feel free to add / change things.

• MSch: I've added some suggestions.