Generalize the gradient energy terms beyond cubic lattice. Ref: [2004 LQ Chen - general gradient energy terms.pdf](https://github.com/AMReX-Microelectronics/FerroX/files/11357732/2004.LQ.Chen.-.general.gradient.energy.terms.pdf)
Generalize the gradient energy terms beyond cubic lattice. Ref:
2004 LQ Chen - general gradient energy terms.pdf