[WIP] Feat: Implement Voronoi-based Constrained DFT (CDFT) with Microiterations

[puzhichen]

This PR introduces an initial implementation of Constrained DFT (CDFT) for Unrestricted Kohn-Sham (UKS) calculations on GPU. The implementation is based on the Voronoi partitioning scheme for defining atomic populations and utilizes the Nested-loop (Microiterations) algorithm for optimizing Lagrange multipliers.

This is a Work in Progress (WIP). The core logic is functional, but further refinement is needed.

Completed:
[x] Basic CDFT logic with charge and spin constraints.
[x] Voronoi projector construction on GPU.
[x] Nested-loop optimization using scipy.
[x] Correct energy definition for variational stability.

To-Do (Upcoming):
[ ] Unit Tests: Add comprehensive tests for N2 dissociation and other benchmark systems.
[ ] Convergence Stability: Modify DIIS implementations.
[ ] Code Cleanup: Optimize memory usage during eval_ao and standardize API interfaces.