Add analytical cone boundaries and validation demos

[yvrob]

Summary

This PR adds native analytical cone support to DEM-Engine, avoiding STL/mesh facets for cone and conical-frustum side contacts.

Changes:

• Add analytical cone object types and DEMExternObj::AddCone / AddConeSegment.
• Support finite cone segments via axial bounds, so frustum sides can be represented exactly; cap planes remain separate analytical planes.
• Add exact sphere-cone contact detection for side, apex, and rim/segment-bound contacts.
• Support inward and outward normals, matching the existing analytical cylinder convention.
• Convert DEMdemo_ConePenetration to use an analytical cone tip instead of mesh/cone.obj.
• Add validation demos:
  • DEMdemo_AnalyticalCone: exact static checks for inward side, outward side, apex, and no-contact control.
  • DEMdemo_AnalyticalConePour: dynamic pouring into corner-down cone, corner-up cone, and frustum-from-below cases.

Validation

• git diff --check
• source /home/codexuser/PBR-DEME_new/dem-engine-env.sh; cmake --build build-cone-smoke --target DEMdemo_AnalyticalCone DEMdemo_AnalyticalConePour -j 2
• GPU exact contact validation on CUDA_VISIBLE_DEVICES=0: passed with 3 active analytical cone contacts and the no-contact control clean.
• GPU dynamic pour validation on CUDA_VISIBLE_DEVICES=0: passed, 225 spheres, 250 frames, ~5 s simulated time.
• ParaView artifact generated from real demo output; cone/cylinder surfaces are display-only, DEM used analytical objects.