Remove gather functions from HeatFluidsDriver

[RonRahaman]

This PR removes the global gather functions from HeatFluidsDrivers. These functions are not used in ENRICO's current coupling scheme.

In ENRICO's earliest coupling scheme, the heat/fluids driver's local fields were gathered into global fields on one neutronics rank. This was feasible for small models, but it's totally infeasible for running large-scale problems. The current coupling scheme never gathers global fields from the heat/fluid driver, and I strongly believe that no future implementation should consider it a viable option.

The changes to HeatFluidsDriver are:

• The functions that return the global fields (centroids, volumes, temperature, density, and fluid_mask) are removed.
• The functions that return the local fields (temperature_local, fluid_local, etc.) are renamed (to temperature, fluid, etc.), since it's no longer necessary to disambiguate them from the functions that return the global fields
• The definition of init_displs is removed. That was used to determine displacements for the global gather. After removing the global gathers, it is not needed.

The changes to Nek5000Driver, NekRSDriver, and SurrogateHeatDriver are:

• Renaming of functions that return local fields, as noted above.

The changes to Comm are:

• The definition of Gatherv is removed.