specify the behavior of the ProvingPeriodDuration function

The [spec](https://github.com/filecoin-project/specs/blob/master/actors.md#commitsector) shows that the [`SubmitPoSt`](https://github.com/filecoin-project/specs/blob/master/actors.md#submitpost) and [`CommitSector`](https://github.com/filecoin-project/specs/blob/master/actors.md#commitsector) methods rely on a function called `ProvingPeriodDuration` which produces the number of blocks in a proving period given a sector size.

We need to understand how to produce the number of blocks in a proving period given a sector size.