fix: Clean up unused imports and variables in mean_field modules

- Remove unused OptimizrError imports in pde_solvers and mod.rs
- Remove unused Array2 import in optimal_transport.rs
- Remove unused Grid and pde_solvers imports in nash_equilibrium.rs
- Fix m_new variable declaration in forward_backward.rs
- Add #[allow(non_snake_case)] for T field/parameter in python_bindings.rs
- Prefix unused hist_cr variable in shade.rs

All changes fix compilation warnings while preserving functionality.

src/mean_field/mod.rs

@@ -62,7 +62,7 @@ pub use nash_equilibrium::*;
 pub use optimal_transport::*;
 
 use ndarray::{Array1, Array2};
-use crate::core::{OptimizrError, Result};
+use crate::core::Result;
 
 /// Configuration for Mean Field Games solver
 #[derive(Clone, Debug)]