[DEM] Add material relations

[jginternational]

trafficstars

Material relations is defined as a list of properties and the join of 2 materials.

See branch /dem/material-relations -> DEM app -> xml/material_relations definition.

Missing:

• [x] Interface CRUD relations
• [x] Writing process prints in the required format with validation (Only print relations used in parts and conditions)
• [ ] Inherited apps can modify parameters
• [ ] Informative window shows missing relations

Developer @jginternational Interested @maceligueta and @KratosMultiphysics/dem

[jginternational]

Working on branch https://github.com/KratosMultiphysics/GiDInterface/pull/810