Add `__repr__` methods to classes

[jonbiemond]

Without __repr__ methods, class instances return hard to distinguish addresses when executed on in the REPL or a notebook.

>>> FEModel3D()
<Pynite.FEModel3D.FEModel3D object at 0x102d11f50>

Add a __repr__ method to make the object easily identifiable.

>>> FEModel3D()
FEModel3D(solution=None)

Files to add __repr__ methods to:

• Analysis.py
• BeamSegY.py
• BeamSegZ.py
• FEModel3D.py
• FixedEndReactions.py
• LoadCombo.py
• Material.py
• MatFoundation.py
• Member3D.py
• Mesh.py
• Node3D.py
• PhysMember.py
• Plate3D.py
• Quad3D.py
• Rendering.py
• Reporting.py
• Section.py
• ShearWall.py
• Spring3D.py
• Tri3D.py
• Visualization.py
• VTKWriter.py