Implement a less computationally demanding version of LYP

[PabloAMC]

Right now GradDDFT implements the original expression for the LYP functional, eq 22 in

Lee, Yang & Parr C. Lee, W. Yang, and R. G. Parr., Phys. Rev. B 37, 785 (1988) (doi: 10.1103/PhysRevB.37.785)

but there is a more computationally efficient one, eq 2 in

B. Miehlich, A. Savin, H. Stoll, and H. Preuss., Chem. Phys. Lett. 157, 200 (1989) (doi: 10.1016/0009-2614(89)87234-3)

This is low priority however.

[jackbaker1001]

This comes for free if we figure out #66