Refactor: reduce FFT calls in Leps2

[LKFEIYI]

Previously, the code seemed to perform unnecessary IFFT (G to R) transformations for each component (x, y, z) and an extra FFT (R to G) for the final result. So I reduced the number of FFT/IFFT calls per iteration from 10 to 6.

Then, the efficiency of the solvation model can be further increased by about 25%. For example, during the scf calculation of Ag-COOH example in #6836 (comment), the time for cal_vel further decreased from 375.72 s to 272.63 s (total time decreased from 667 s to 566 s, GNU toolchain, 7950x, mpirun -n 16).

I will appreciate it if you can check if they are correct and safe.

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[dyzheng]

LGTM