New features for the derivatives of PBC Gaussian density fitting

[sunqm]

This PR implements new features and improvements for PBC GDF gradient code.

• Adds long-range Coulomb contributions to the nuclear gradients of the PBC GDF integrator for RHF and KRHF
• Adds nuclear gradients for the PBC GDF integrator in UHF and KUHF
• Updates the convention of the get_ej_ip1 and get_ek_ip1 functions in the PBC aft module to return energy derivatives per unit cell. Previously, these functions returned nKpts / 2 * per-cell value.
• Refactors other relevant PBC gradient modules