SurrogateHeatDriver doesn't know about guide tubes

[paulromano]

Right now, when we specify <n_pins_x> and <n_pins_y> for the heat/fluids surrogate, it doesn't account for guide tubes, so we end up mapping and doing data transfers on regions that don't make sense.

[aprilnovak]

Technically the temperatures of the guide tubes will be affected by the coolant temperature (no heat source = guide tube temp matches fluid temp), so there is some feedback, but it's probably very small and not super important to include in the surrogate anyways.

I think we could add a new parameter like guide_tube_indices to provide (x, y) integer indices for pins that are guide tubes and therefore have a different geometry and no heat source. We can include feedback with some modifications to set_heat_source (because we won't set up tallies for the guide tubes). Or, we could skip the feedback and just omit them from the mapping and not bother solving T/H on them. Which would you prefer?