Add ScenarioMarketDataId for FxRatesArray

[cjkent]

This will allow FxRatesArray to be accessed directly. This allows the caller to use the efficient, boxing-free bulk conversion method FxRatesArray.convert

[cjkent]

In order to integrate with the recent changes to the market data API this should probably include a new method ScenarioFxRateProvider.fxRatesArray(baseCurrency, counterCurrency). This would allow the array of rates to be created by triangulation if necessary and possibly cached.