[PLAT-3130] Renaming some expiry calculator methods so that it's clear that they are to be used for one sort of instrument only, and deprecating methods that hard-code behaviour. Added a method that takes a period as well as the expiry number to allow the type of future to be specified precisely
[PLAT-3129] Equity index future option surface instrument provider. This provider uses the new functionality to specify what sort of expiries the number refers to