Add validation to prevent steadystate solver with multiple sediment fractions

[Copilot]

The steadystate solver is not compatible with multiple sediment fractions, but this constraint was not enforced, allowing invalid configurations to run.

Changes

• Added validation in check_configuration() to detect and reject steadystate solvers (steadystate, steadystatepieter) when grain_size contains multiple fractions
• Validation triggers after input parsing with clear error message directing users to either use single fraction or switch solvers (e.g., trunk, pieter)

# Now raises ValueError at initialization:
config = {
    'solver': 'steadystate',
    'grain_size': [225e-6, 300e-6],  # Multiple fractions
    ...
}
# Error: The steadystate solver is not compatible with multiple sediment fractions.

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