[Mechanics_OG] Implement agent container

[joseestragues]

Summary
Add container for managing mechanical agents and ID assignment.

Steps

1. Create mechanics/micromechanics/include/mechanical_agent_container.h.
2. Have a look at biofvm agent_container

Acceptance Criteria

Adding agents increases size

Verification
Temporary test or simple main run before formal tests.

Dependencies
Mechanical agent.

Follow-ups
Spatial index build step.