Simplify ninjafoam case reuse

[nwagenbrenner]

Right now the existing case option is not entirely intuitive. Choosing a case directory might be confusing to some users. And if the DEM has moved, the user has to manually load it. To simplify things, we could zip up the NINJAFOAM_* case with the DEM at the end of the simulation and use these zip files for existing cases. We can give the zip files a custom extension so we can auto-detect them in the GUI file browser.

[ksshannon]

Another case for #7. Gives devs more control. Just write full files to the case (which is a single file so less confusing).