Use clearer parameters

[sembrestels]

Use halfLife, maxThreshold and minThreshold instead of alpha, beta, and weight. Change the name of _minThresholdStakePercentage for minEffectiveSupplyPct.

Also, use PPM = 1e6 instead of D = 1e7 and ONE_HUNDRED_PERCENT = 1e18 to make it standard with Marketplace.

[willjgriff]

I think minThresholdStakePercentage should be called minActiveStakePercentage.