Generalized non-hourglass formulation for ULSPH solid dynamics

[Shuaihao-Zhang]

The generalized non-hourglass formulation for ULSPH solid dynamics.

Constitutive models include elasticity and plasticity (with/without hardening) are considered.

Two cases are uploaded for testing: test_2d_oscillating_beam_ULGNOG for elastic dynamics, and test_3d_taylor_bar_UL for plastic dynamics.

Regression tests are added.