Add batch FFT functionality and corresponding GTest cases for batch FFT operations

[A-006]

What's changed?

• Implement batched FFT for the GPU-based PW basis.
• Add GTest cases for the operators:1.set_real_to_recip_op2.set_recip_to_real_op3.veff_op(npol=1)4.veff_op(npol=2)5.set_3d_fft_box_op
• Utilize GTest to ensure all operators are fully functional.
• Test the batched FFT implementation: Compare performance/results with single FFT operations.

Attention

The operations preceding and following the FFT are currently available for use but not amenable to optimization.

[A-006]

Temporarily close it for optimization.