Draft implementation of LOD API in OCCT:

Draft implementation of LOD API in OCCT:
- implemented interfaces for LOD at OpenGl, Graphic3d and MeshVS levels;
- added LOD managers at Graphic3d and OpenGl levels;
- simple mechanism of LOD switching is implemented in LOD manager;
- distance to COG metric is temporarily used;
- the metric could be redefined using custom LOD selectors;
- LODs are added to MeshVS_Mesh in Compute method and are listed as its data sources;
- added commands meshgenlod and meshlod to generate LODs for mesh and its topological representation
  and bind them to its OpenGl structures.