Modify the URDF model for meshes paths

## Objective
The goal is to make the human model URDF agnostic to local directory paths for meshes. Currently, the mesh files in the URDF model are referenced using a fixed local directory path. This requires modifying the URDF file on every computer or for every user. Instead, we aim to update the URDF to use a `package` variable, which will:

1. Allow mesh paths to follow a standard format, such as:
```xml

```
2. Require the `package` variable to be set as an environment variable by the user. This variable should point to the `meshes` folder inside the `human-model-generator` repository.
3. Enable interchanging URDF models without requiring any modifications to the file.