Currency Exchange Maximum Rate

You are given a list of currency exchange rates represented as tuples (from_currency, to_currency, rate) and a starting currency start. Your task is to compute the maximum amount of the start currency you can obtain by performing any sequence of conversions (including zero conversions). You may convert through intermediate currencies and may use each rate in either direction (if A->B has rate r, then B->A has rate 1/r). Return the maximum multiplier achievable relative to the starting amount; if no conversion path exists that improves the amount, return 1.0. If the start currency is not present in any rate, return 1.0.