[Mechanics_OG] Add simulation parameters struct

[joseestragues]

Summary
Introduce simulation_parameters struct centralizing mechanics tunables.

Steps

Create mechanics/physicell/include/simulation_parameters.h.
Define struct SimulationParameters with fields from physicell xml settings file related to mechanics (validate with paraPhysiCell repo)
Add short comments (units, purpose) for each field.
Keep the most important parameters in environment.h: dt, agent_types_count, vector of simulation_parameters

Acceptance Criteria

Struct compiled; defaults set.
Referenced by environment constructor.

Verification
Build and ensure no undefined references.

Dependencies
Library skeleton.

Follow-ups
Used in force model and integration logic.