Fix error in CGNS writing

[rois1995]

Hello everyone,

I encountered an error in the writing of a CGNS file when no nodes are associated to a given rank. It usually happens when small grids are employed and the simulation is performed on more than one core. have uploaded at the following link the mesh and the config used to reproduce the error.

https://polimi365-my.sharepoint.com/:f:/g/personal/10507725_polimi_it/Ekl88a3uOKhMu4uCLRy3tHIBNNo6_3oMTNiZq97NB6ztDw?e=Hu8pqe

Notice that, if only one heatflux marker is selected to print the results onto, then it does not work when 2 or more cores are used. On the other hand, if both of the heatflux markers are used then it works in parallel.

To solve this issue I just added a check on the number of local points in the function CCGNSFileWriter::WriteField.

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[pcarruscag]

Add the case to our regressions please

[rois1995]

I think I managed to do it, please let me know if I did something wrong