Enahancement; Shift `period` to `modifyRailLockup`

we should maybe move the period parameter from `modifyRailLockup` to `modifyRailPayment`, fixed lockup is completley separate and is not linked to rate and period, while mostly we have used `rate * period` together to get the payments, it would be better to handle these together only rather than having it with fixed lockup which can become little confusing.