Remove a few IExtrapolator interfaces

The IExtrapolator seems to have > 20 interface methods.

• Quite of few of them are not actually used anywhere
• A few of the unused ones provide similar signature/functionality as the current Impl methods (Added in MT)
• But they lack the Cache which was added for MT (to avoid internal state). So they end up creating a cache and just calling the impl
• This is more header changes rather than .cxx changes , i.e is just unused interfaces (Do not try to really touch the core implementation here)

mentioning @sroe, @amorley , @asalzbur in case it actually turns out that indeed we can remove a few of them ...