Add 3D MMS collection: sinusoidal and linear Neumann & Dirichlet problems (Q1 hexahedra)

[Fimache]

Three 3D manufactured solutions for hexahedra:

• mms-lin-neumann.py : linear MMS with Neumann BC; superconvergence observed

• sin-3d.py : sinusoidal with Dirichlet BC

• sin_neumann.py : sinusoidal with Neumann BC

• L2 and H1 error norms included

[th-skam]

Before diving in, the main thing to do here is to adopt what has been done in #16 with the RegularGridTopology and the traction BC integration.
Ideally, you should reuse the functions we created in that file and add the quadrature rules that you need there.

[th-skam]

One issue I see is the amplitude of the wave function. The domain spans [0,1] in all directions. The function we're using is of the type:
u_x,y,z = sin() * cos() * sin() etc.

The maximal displacement at the center of the domain equals 1 and this pushes the internal points outside.

The manufactured solution should be something like:
u_x,y,z = A * sin() * cos() * sin().
The amplitude should scale appropriately.

[th-skam]

Let's merge this. Reviewing the functions' regularity and appropriateness should be the topic of future PRs. Too many open questions for one PR.

Results are good: