Add support for more time formats in rate_inv_fmt

[george-gca]

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When setting rate_inv_fmt in format_meter function, it would be nice to use different time formats when unit_scale is True (maybe create a format_rate function to use here instead of format_sizeof?).

This way, we could see information like 1.35min/it, 8.55h/it, or even 1.5d/it.