Cinema Export Plugin: camera grid doesn't handle multiblock data

[EthanS94]

@dhrogers @JonasLukasczyk

I tried exporting with the new plugin in paraview 5.13.1. I found this with a different dataset, but it can be recreated with the example dataset bake.e. When creating a camera grid on a multiblock dataset, it looks like there is one camera grid created per block. There should only be one camera grid for the whole dataset.

To reproduce:

• Open paraview 5.13.1
• Load Cinema Export Plugin
• Load bake.e dataset
• Add Cinema Camera Grid filter to bake.e dataset

Running merge blocks before trying the camera grid works. One fix is to not allow running cinema filters on multiblock datasets, and document this in the GUI + Export Documentation.