Quad4 and improvemenets in Quad4R

[saullocastro]

Quad4R was the only available quadrilateral finite element. Despite beinf very efficient, the Quad4R suffered from:

• hourglassing and the current hourglass control method is not generally robust
• drilling stiffness was coupled with the in-plane rotation, coupling the DOFs rz, u and v

With this pull request the following is implemented:

• Quad4 element with mixed integration that eliminates hourglass modes
• K6ROT in its pure form, directly entering the diagonal terms of the stiffness matrix corresponding to the rz DOF, without coupling with u and v