Basic oversampling

[jamwaffles]

Adds set_oversampling method to SubDevice as a stopgap solution to configure oversampling. The proper solution will look like some sort of ESI file parsing but that's a long way off. Example usage for EL3702:

const OVERSAMPLE_MUL: u8 = 8;

for mut subdevice in group.iter_mut(&maindevice) {
    if subdevice.name() == "EL3702" {
        log::info!("Found EL3702");

        // Configure oversampling for both input channels
        subdevice.set_oversampling(&[
            (0x1a00, OVERSAMPLE_MUL),
            (0x1a80, OVERSAMPLE_MUL),
        ]);

        subdevice.set_dc_sync(DcSync::Sync01 {
            sync1_period: Duration::from_micros(
                TICK_INTERVAL.as_micros() as u64 * OVERSAMPLE_MUL as u64,
            ),
        });
    }
}