Support for spawn constraints in animats

[jarreguit]

Currently, spawn options in FARMS assume that arena elements have a fixed base, while animats are expected to have a free base. However, in certain scenarios, it is desirable for animats to spawn with a fixed base or to be constrained to move within a plane (e.g., when simulating 2D locomotion). To address this need:

• The farms_core package should be extended to support additional animat spawn options. These options will allow specifying whether animats have a fixed base, planar constraints, or free movement. See the related issue in farms_core.

• The farms_mujoco package should implement these spawn options by modifying the setup of animats in the MuJoCo simulation. This would allow constraints to be applied programmatically during initialization, improving flexibility for a variety of simulation setups.

This issue tracks the addition of spawn constraints for animats and the associated changes in farms_mujoco.