Add parallelization to the smooth covering grid.

[AnatoleStorck]

PR Summary

I've added functionality for generating a smooth covering grid by loading the data chunks in parallel using @cphyc #5218 's piter() function. I've also replaced the parallel for loop of the regular covering grid. This PR assumes that the functionality of #5218 is merged. The functionality has been confirmed to work both on sample dataset and a complex simulation.

[cphyc]

For reference, this needs to be merged after #5218 gets merged (if it ever gets merged :) ).