Function feature_row 

pub fn feature_row(
    gene: &str,
    modality: &str,
    channel: &str,
    subunit: Option<&str>,
) -> Box<str>

Format a feature row. Pass subunit = None for a gene-level (pooled) row {gene}/{modality}/{channel}, or Some(site_or_component) for a sub-gene row {gene}/{modality}/{subunit}/{channel} (channel innermost). The subunit must not contain / (sites use the single-base chr:pos, components are integers), so the row round-trips through parse_feature_row.