Fix BoundsError in upwind_difference for nonuniform grids (Fixes #340)

[utkuyilmaz1903]

This PR resolves Issue #340 where providing a nonuniform grid (as an AbstractVector) to the UpwindScheme resulted in a BoundsError.

Cause of the Bug:
In _upwind_difference for DX <: AbstractVector, the D.offside value defaults to 0. This caused the boundary check (II[j] <= D.offside) to fail at the lower boundary (II[j] == 1). Consequently, the algorithm incorrectly executed the interior stencil logic, attempting to access index 0 and throwing a BoundsError.

Solution:
Replaced the reliance on D.offside with a calculated actual_offside = D.stencil_length - 1. The boundary checks now correctly identify when the stencil approaches the domain edges, appropriately applying the boundary coefficients (low_boundary_coefs and high_boundary_coefs) without out-of-bounds indexing.