Function ensure 

pub fn ensure(
    dir: &Path,
    e: &ExpertLayout,
    bits: u32,
    force: bool,
) -> Result<Layout>

Make sure the bits-bit store exists next to experts.bin, building it from the 4-bit records if it is missing, the wrong size, or in the older code-at-a-time layout. Returns the layout either way.

force rebuilds an existing store (–repack).