update freesurf to 3D in uwg

[NengLu]

1. update the free surface function in UWGeodynamics to a 3D version (see details in _freesurface.py).
2. add an example 1_23_05_FreeSurface_3D_Relaxation in docs/UWGeodynamics/examples/.
3. add some images for the 2D free-surface examples.

[julesghub]

• Create png image for the top of the example 1_23_05_FreeSurface_3D_Relaxation
• Remove output viz from the example.
• Add documentation in CHANGES.md (method description, where are the models, etc.)

[julesghub]

We will use this in uw2.16 along with some dependency updates.