Function check_inputs 

fn check_inputs(
    inputs: &SeparationInputs<'_>,
    metadata: &ScanMetadata,
    algorithm: SeparationAlgorithm,
    n: usize,
) -> Result<(), PipelineError>

Check what the separation algorithms would otherwise assert! on, so bad input is an error rather than a panic: map sizes, the multi-echo magnitude’s echo count, Decompose’s minimum of three echoes, and an empty mask.