Fix failing unit tests related to the position map

[Maxwell-Rosen]

Bug fix

Summary

Description:

Mana informed me that the twist-shift BC unit test was failing and it seemed to be caused by the position map.

I found this quite strange because when I ran these unit tests on perlmutter GPU, they were just fine on main.

In any case, I did see a potential issue with memory handling regarding how the gk_geometry modules create a null position map if none is provided. This memory handling could lead to different behavior on different machines (although here, both Mana and I were testing on Perlmutter GPU). Even though I can't reproduce the same failure, I cleaned up the code by intentionally allocating a null position map in the unit tests and passing that to the geometry generators.

Solution

Create a null position map in each unit tests.
Remove code in the gk_geometry generators to detect if no position map is passed.
Removed unused "flags" variable from the position map.

Automated testing: unit tests are updated. position_map tests are updated accordingly.

Community Standards

• Documentation has been updated.
• My code follows the project's coding guidelines.
• Changes to layer/zero should have a unit test, e.g., core/zero.

Testing:

• I added a regression test to test this feature.
• I added this feature to an existing regression test.
• I added a unit test for this feature.
• Ran make check and unit tests all pass.
• I ran the code with make valcheck, and it is clean.
• I ran the code through computer-sanitizer on GPU, and it is clean.
• I ran a few regression tests to ensure no apparent errors.
• Tested and works on CPU.
• Tested and works on multi-CPU.
• Tested and works on GPU.
• Tested and works on multi-GPU.

Additional Notes

I would like @manauref to run the appropriate tests before this is merged. I am unable to reproduce the original error, so I cannot verify that these modifications fix the issue.