Add elements to file format

[maierbn]

Rather than discrete particle our simulation uses a continuum approach where the variable fields are comprised of a linear combination of basis functions. These basis functions have local support and are organized in elements. The coefficients of the linear combination are the degrees of freedoms (dofs) which are located at nodes.

We therefore need in the file format

• [ ] potentially multiple field variables
• [ ] each field variable has multiple components
• [ ] each component has one entry for each dof with one version or two entries for dofs with two versions
• [ ] multiple dofs are grouped at each node. The number of dofs per node is constant per field variable
• [ ] elements contain a specified set of nodes each, e.g. 2D 4-node elements, 9-node elements, 3D 8-node elements, 27-node elements should be possible. A node is shared by (multiple) adjacent elements.
• [ ] there are versions of dofs where a dof has multiple values for different of the adjacent elements

[maierbn]

Then we have different meshes,

• the 3D continuum mechanics mesh which contains 3D elements (potentially 2 different types of elements)
• the 1D fibre meshes with 1D elements