[Mapping.Linear] Add applyJT for matrices for IdentityMultiMapping

[alxbilger]

Based on #6032

The goal was to verify that the two following scenes produce the same behaviour:

1. Two objects in the same solver. No mapping involved

<Node name="root" gravity="0 -9.81 0">

    <Node name="meshes">
        <StringMeshCreator name="string_1" resolution="10"/>
        <StringMeshCreator name="string_2" resolution="8"/>
    </Node>

    <EulerImplicitSolver/>
    <SparseLDLSolver template="CompressedRowSparseMatrixMat3x3d"/>
    <GlobalSystemMatrixImage/>

    <Node name="string_1">
        <MechanicalObject name="state" src="@/meshes/string_1" translation="0 0 0.5" />
        <MeshTopology src="@/meshes/string_1" />
        <MeshSpringForceField name="spring" drawMode="1" linesStiffness="1000"/>
        <FixedProjectiveConstraint indices="0"/>
        <UniformMass vertexMass="2"/>
    </Node>

    <Node name="string_2">
        <MechanicalObject name="state" src="@/meshes/string_2" translation="0 0 -0.5" />
        <MeshTopology src="@/meshes/string_2" />
        <MeshSpringForceField name="spring" drawMode="1" linesStiffness="1000"/>
        <FixedProjectiveConstraint indices="0"/>
        <UniformMass vertexMass="2"/>
    </Node>
</Node>

2. Two objects. The mass is mapped, applied on the concatenation of both objects.

<Node name="root" gravity="0 -9.81 0">

    <Node name="meshes">
        <StringMeshCreator name="string_1" resolution="10"/>
        <StringMeshCreator name="string_2" resolution="8"/>
    </Node>

    <EulerImplicitSolver/>
    <SparseLDLSolver template="CompressedRowSparseMatrixMat3x3d"/>

    <Node name="string_1">
        <MechanicalObject name="state" src="@/meshes/string_1" translation="0 0 0.5" />
        <MeshTopology src="@/meshes/string_1" />
        <MeshSpringForceField name="spring" drawMode="1" linesStiffness="1000"/>
        <FixedProjectiveConstraint indices="0"/>
    </Node>

    <Node name="string_2">
        <MechanicalObject name="state" src="@/meshes/string_2" translation="0 0 -0.5" />
        <MeshTopology src="@/meshes/string_2" />
        <MeshSpringForceField name="spring" drawMode="1" linesStiffness="1000"/>
        <FixedProjectiveConstraint indices="0"/>
    </Node>

    <Node name="mass">
        <MechanicalObject name="state"/>
        <IdentityMultiMapping input="@../string_1/state @../string_2/state" output="@state"/>
        <UniformMass vertexMass="2"/>
    </Node>
</Node>

Both scene should produce the same result. Except that the scene with the mapping has a problem. See the system matrix:

The mass matrix is missing. It's because IdentityMultiMapping does not implement applyJT for matrices.

With this PR, the system matrices is now:

Note:
IdentityMultiMapping is not available in <Vec3,Vec2>, but I made sure that the current implementation is compatible with such templates. For example, I tried the following scene:

<Node name="root" gravity="0 -9.81 0">

    <Node name="meshes">
        <StringMeshCreator name="string_1" resolution="10"/>
        <StringMeshCreator name="string_2" resolution="8"/>
    </Node>

    <EulerImplicitSolver/>
    <SparseLDLSolver template="CompressedRowSparseMatrixMat3x3d"/>

    <Node name="string_1">
        <MechanicalObject name="state" src="@/meshes/string_1" translation="0 0 0.5" />
        <MeshTopology src="@/meshes/string_1" />
        <MeshSpringForceField name="spring" drawMode="1" linesStiffness="1000"/>
        <FixedProjectiveConstraint indices="0"/>
    </Node>

    <Node name="string_2">
        <MechanicalObject name="state" src="@/meshes/string_2" translation="0 0 -0.5" />
        <MeshTopology src="@/meshes/string_2" />
        <MeshSpringForceField name="spring" drawMode="1" linesStiffness="1000"/>
        <FixedProjectiveConstraint indices="0"/>
    </Node>

    <Node name="mass">
        <MechanicalObject name="state" template="Vec2"/>   <!-- Note here the 2d -->
        <IdentityMultiMapping input="@../string_1/state @../string_2/state" output="@state"/> <!-- not available, but I compiled it for the experiment -->
        <UniformMass vertexMass="2"/>
    </Node>
</Node>

The system matrix is:

It is rank-deficient, but it is expected (the mass is computed in 2d, not in 3d. One component is missing). So it works as expected.

---

By submitting this pull request, I acknowledge that
I have read, understand, and agree SOFA Developer Certificate of Origin (DCO).

---

Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).