Implement iterator over 2D pixel coordinates

[horizon-blue]

Closes MET-56

Summary of Changes

This PR implements a PixelCoordRange class, which defines .begin() and .end() methods compliant with C++'s range-expression. Instead of using two Iterator to denote the range, the end() returns a special Sentinel object, which requires less bookkeeping information (e.g. it doesn't need to keep another copy of the pixel start & end location). You can read more about C++20's Iterator Sentinel feature here if interested :).

With this new class, we can now iterating over 2D pixels using range-based for loop (as we wished for in Slack), i.e.,

auto pixel_ranges = PixelCoordRange{ i_start, i_end, j_start, j_end };
for (auto pixel_coords : pixel_ranges)
   // do something

Test Plan

This new pixel coordinate generator replaces the need to have a dedicated Intrinsics.get_ray_directions_kernels method (which is removed in this PR). As such, camera_test.cu has been updated accordingly to show & test that range-based for loop works as expected.

As always, to run the tests:

pixi run ctest

[linear]

MET-56 Implement an iterator over pixel coords

[horizon-blue]

Okay, based on offline discussions, I'm going to merge this now so we can keep moving forward :).