Expose certain PP5 context properties in PP5EnrollTransform

@JordanCheney @mmtaborsky While the default PP5 settings are good for getting the maximum recognition performance under all conditions, they are non-ideal when enrollment speed is a concern. This PR allows certain context properties to be set in the algorithm string that are particularly relevant for changing the speed/accuracy tradeoff.
It's not quite `ln(IPD)` though right? `minSize 5 ~= 29 pixels`, but `ln(29) ~= 3.4`. My hesitation for exposing pixels in the interface instead was that minSize is an integer, and depending on rounding, changing IPD won't necessarily change minSize.