Implement hourglass correction for quad4 elements with 1st order Lagrange shape functions

[dschwen]

Motivation

For explicit dynamics we'd like to switch to under integrated (constant strain) elements. With one QP the number of material model evaluations is cut by a factor of four. However, under integration introduces zero-energy modes (hourglass modes). Stabilization terms exist to suppress these modes.

Design

Add a kernel to compute the nodal forces resulting from the hourglass correction energy.

Impact

Added functionality.