Make writing point IDs optional

Summary

Point IDs are needed to reconstruct a parallel grid. Each vertex must have a unique identification in order to associate them properly when reading the grid in parallel, e.g. with a GridFactory for ALUGrid.

But, reading parallel grids must be enabled explicitly. Writing the point IDs is an expensive operation (a global unique id must be computed, e.g., by using a GlobalIndexSet resulting in a lot of communication). Reading parallel grids only works with ALUGrid. The files containing point IDs are larger than the ones without these IDs. This MR thus makes the writing of point IDs thus optional.

Example

Point IDs can be enabled in the data collector. Currently, only the ContinuousDataCollector supports PointIDs.

Vtk::ContinuousDataCollector<GridView> dataCollector(gridView, true); // point IDs explicitly enabled
VtkUnstructuredGridCreator<GridView> writer(dataCollector);