Adds a view method for getting current chi.

[MicahZoltu]

This change makes it so a contract with a view function can know what chi would be in this block. This is useful for contracts that have view functions that need to return values denominated in DAI when looking at DSR balances.

An example use case for this is a token contract (DAI-GoUp) that wraps DAI held in DSR. The balanceOf function of such a contract (which is view) needs to be able to calculate the current balance of an account denominated in DAI. In order to do this, it must calculate what chi would be if drip was called this block, but without actually calling drip (because that is not a view function).

[MicahZoltu]

Open to whatever changes to function/variable naming you all want.

[MicahZoltu]

We could change this method to public and use it in drip(), but the style in this repository appears to prefer duplication over having public methods so I went with that in an attempt to follow the style found here. Can change if desired.

[DaiFoundation-DevOps]

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