Add AFLIP integration scheme
This branch adds the Affine-augmented FLIP (AFLIP) integration scheme as described in [1] (as well as the AFLIPX strategy). Only a few changes were necessary since the APIC scheme was already implemented.
[1]: Yun (Raymond) Fei, Qi Guo, Rundong Wu, Li Huang, and Ming Gao. 2021. Revisiting integration in the material point method: a scheme for easier separation and less dissipation. ACM Trans. Graph. 40, 4, Article 109 (August 2021), 16 pages. https://doi.org/10.1145/3450626.3459678