feat: add unfinished_candle method

[ivaaaan]

Another problem I have faced, is that there is no way to get the latest candle from the aggregator.

Input data:

timestamp,price,size
1612137602564000,27334.25,0.00289
1612137664747000,27298.5,0.00909044

TimeRule and AlignedTimeRule will trigger on a first trade, and will return the candle, but there is no way to get the second candle.

I propose to add a new state for candle - finalized, and method close to aggregator. This way, user can easily retrieve the latest candle from the aggregator.

Let me know if there are other ways to achieve this.

[MathisWellmann]

I'd prefer simply adding a unfinished_candle getter method which provides even more flexibility compared to simply closing an Aggregator.
Its a good feature proposal overall, enhancing the flexibility.
We should be careful to ensure that its clear to a library user that accessing a Candle through a method like this is not recommended as it does not respect the AggregationRule. But for more advanced use cases this is useful

[MathisWellmann]

Suggested change

	/// Close current active candle and return it
	///
	/// # Returns:
	/// Last active candle, or None if candle was already finalized
	fn close(&mut self) -> Option<Candle>;
	/// Get a reference to an unfinished `Candle`.
	/// Accessing a `Candle` using this method does not guarantee that the `AggregationRule` is respected.
	/// It is generally advised to call `update` instead and use the resulting `Candle` if its `Some`.
	fn unfinished_candle(&self) -> &Candle;

[ivaaaan]

I agree, that's what I ended up doing - implemented my own aggregator with method like this. Will update the PR

[ivaaaan]

@MathisWellmann updated :)