BPM and WPM fixes for odd grids

[EkaterinaBolotskaya]

1. BPM asymmetry - commit 1

BPM simulations are asymmetric on even grids and produce an error on odd grids.
BPM fix - “build frequency list correctly.”

Bug demonstration

Added a PSF profile into the lenses test file: https://diffractio.readthedocs.io/en/latest/source/examples_scalar/lenses.html XZ lens: BPM scheme section

Before the fix:
Asymmetric results on an even grid, error on an odd grid.

Changed the X resolution to (even):
x0 = np.linspace(-diameter / 2, diameter / 2, 2*1024)

Changed the X resolution to (odd):
x0 = np.linspace(-diameter / 2, diameter / 2, 2047)

After the fix in scalar_fields_XZ
Symmetric and no errors.

Changed the X resolution to (even):
x0 = np.linspace(-diameter / 2, diameter / 2, 2*1024)

Changed the X resolution to (odd):
x0 = np.linspace(-diameter / 2, diameter / 2, 2047)

I DID NOT TEST THE EQUIVALENT FIX I MADE IN scalar_fields_XYZ

2. WPM asymmetry on odd grids

This one was identified by Lisa Gavronskiy (lisa.gavronskiy@irradiant.tech, https://github.com/lisagavronskiy)
WPM fix - “put spectrum back in the right order before inverse FFT.”

Same example used.
Resolution - odd:
x0 = np.linspace(-diameter / 2, diameter / 2, 2023)
Method BPM -> WPM

Before:

After:

I DID NOT TEST THE EQUIVALENT FIX I MADE IN scalar_fields_XY

Testing

pytest output is the same before and after fixes:

===================================================================================== short test summary info ====================================================================================== 
FAILED tests/test_scalar_fields_X.py::Test_Scalar_fields_X::test_RS_amplification - OSError: [Errno 22] Invalid argument: 'D:\\GitHub\\diffractio\\tests_results_2026-03-24_11\\scalar_fields_X\\test_RS_amplification_RS*3.png'
FAILED tests/test_scalar_fields_X.py::Test_Scalar_fields_X::test_MTF - TypeError: Scalar_field_X.MTF() got an unexpected keyword argument 'kind'
FAILED tests/test_scalar_fields_XY.py::Test_Scalar_fields_XY::test_MTF - TypeError: Scalar_field_XY.MTF() missing 2 required positional arguments: 'fx' and 'fy'
FAILED tests/test_scalar_masks_XY.py::Test_Scalar_masks_XY::test_super_gauss - AttributeError: 'Scalar_mask_XY' object has no attribute 'super_gauss'
FAILED tests/test_scalar_masks_XZ.py::Test_Scalar_masks_XZ::test_discretize_refractive_index - ValueError: operands could not be broadcast together with shapes (256,) (7,)
FAILED tests/test_utils_math.py::Test_utils_math::test_distance - numpy.exceptions.AxisError: axis 1 is out of bounds for array of dimension 1
FAILED tests/test_utils_math.py::Test_utils_math::test_amplitude2phase - AssertionError: test_amplitude2phase
================================================================= 7 failed, 238 passed, 3 skipped, 3 warnings in 275.59s (0:04:35) =================================================================