[FEM] Add component for source term integration based on new FEM framework - constant source terms

[th-skam]

PR Based on #6225

The PR adds component with which to integrate source terms using the FEM framework ported from Elasticity.

There is a central FEMSourceTermIntegrator component placed in the scene responsible for gathering all source terms present, and integrating them into loads. Added examples demonstrate this.

I added 3 distinct classes that can implement the following loads:

The type of the term	Formula	User input
Pressure	$L_p = \int_{S} \vec{v} \cdot (-p \vec{n}) dS$	The pressure $p$ on the boundary
Stress	$L_{\sigma} = \int_{S} \vec{v} \cdot (\sigma \vec{n}) dS$	The stress tensor $\sigma$
Force	$L_{t} = \int_{S} \vec{v} \cdot \vec{t} dS$	The force density $t$ directly

The last object can also mimic body forces (gravity etc).

Currently in SOFA, these loads were spread out among various components, such as SurfacePressureForceField, QuadPressureForceField, EdgePressureForceField, TrianglePressureForceField among others. Each component implements their own integration loop and uses geometric data (such as normals) directly from its linked topology.
The loop is unified in the FEMSourceTermIntegrator and data such as normals are computed from the mapping jacobian of the reference domain. Extending them to quadratic elements is straightforward by using a quadratic topology in the scene; no more code extensions.

The scenes that have been added demonstrate how the new source term classes can duplicate the existing functionality.
In this PR, the loads are applied on the reference geometry. That's to simply introduce them and I'll add the ability to follow the geometry in subsequent PR.

Fixes #6166

[force-full-build]
[with-all-tests]

[bakpaul]

I am not a FEM expert so I don't feel confident to review this PR. Nevertheless I checked to make sure this is compatible with higher order element and it seems ton be the case. So it is fine by me given that @alxbilger and/or @hugtalbot review it.

Could you add an example ?