Remove allocations in interpolants(grid::RectangleGrid, x::AbstractVector) and interpolate using view

[himanshugupta1009]

1. Modified the interpolants function to return a view of grid.index and grid.weight instead of slicing it (gets rid of allocations).
2. Modified the interpolate function to avoid allocation (data[index] was causing allocations).

Performance comparison

Before these modifications

After these modifications

If there are any issues with it, please let me know. Thanks!

[tawheeler]

Overall this change looks great to me. I'm surprised dot was contributing to allocations, but the data does not lie. Thank you!

Could the indentation of the code be adjusted to match with the existing code? Usually that delta is a matter of tabs vs. spaces.

[himanshugupta1009]

@tawheeler I think the allocation was due to the argument that was being passed to the dot function (data[index]).

I didn't realize the indentation mismatch before. Thank you for pointing it out. I have replaced tabs with spaces and it now matches the existing code.

[zsunberg]

Looks good to me! There might be a better way to do the for loop, e.g. with sum, but I think we should just merge this!

[zsunberg]

This is a possible alternative for the for loop

[zsunberg]

Would this work?

Suggested change

	v = 0.0
	for (i,data_ind) in enumerate(index)
	v += data[data_ind]*weight[i]
	end
	return v
	return dot(view(data, index), weight)

[himanshugupta1009]

@zsunberg Yes, this works too. It just slows down the computation by around 15 ns (checked multiple times). If that is acceptable, I can make this change. Let me know.

Before making the change suggested by Dr. Sunberg

After making the change suggested by Dr. Sunberg

We can also use the sum function like you mentioned but it also causes a similar increase in total time.

sum( i->data[index[i]]*weight[i], 1:length(index))

[zsunberg]

Either one is fine. dot is easier to read, but the for loop performs slightly better.