Define the groups on the model of blocks

A group is characterized by the three following components:

• its type
• its name
• its elements

The six following classes define all the kind of group:

• group3d for groups of hexahedra,
• group2d for groups of facets,
• group1d for groups of segments,
• group0dVertices for groups of nodes on the edges,
• group0dFaces for groups of of nodes on the faces,
• group0dBlocks for groups of nodes in the blocks,

See TUI scripts and GUI part.