xtaohub/Sayram-2D: v.1.1.0

Improved separation between the Solver and Equation classes. Boundary conditions are now handled entirely within Equation, simplifying extensions and customization. Replaced the iterative Eigen solver with SparseLU. This avoids reliance on tolerance-based convergence (setTolerance). Users who prefer an iterative solver can switch back with minimal changes. Introduced a new Grid2D class to support non-uniform grids in a clean and modular way. Added a new ZeroFlux boundary condition. Provided an additional test case, Albert_Young_LC, demonstrating an alternative loss-cone boundary condition.