MovingWindow: Create Interface for requesting data and handling the data copy behaviour

[matthias-wende-frequenz]

When asking the MovingWindow for a subwindow we might return a slice of the buffer, that means a reference to the underlying numpy based ringbuffer.
There are cases, e.g. when dealing with small window sizes and high sample rates, where the data might get updated from the data stream before the requested subwindow has been processed.

As a solution we want to tell the MovingWindow to return a copy of the data instead of a reference. In order to do that we need to add an interface, such that users can control copy behavior.

We might implement a method for the MovingWindow as follows.

window(
	self,
	start: datetime,
	end: datetime,
	fill_gaps: Optional[Quantity] = None,
	force_copy: Bool = False
)

This method should return a sorted numpy array that holds the resampled data from start to end and fills the gaps with the Value that is provided by fill_gaps.

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Summary of the (ongoing) work related to this and what I see essential for v1.0.

Essential for this issue:

• For v1.0:

  • Treat NaN as gap in ring buffer #641
  • Add capacity, oldest and newest timestamp to moving window #598
  • Fixes on copy behavior in ring buffer window method #638
  • Support int indices and slice index behavior in ring buffer and moving window #668
  • Fix single element access for moving window #672
  • Fix getitem magic in moving window #676 (If not ready for v1.0 getitem magic should be removed before the release)

• Post v1.0?

  • Add option to impute missing values in window method for moving window #669.
  • Inconsistent gap treatment in ring buffer #646

Related to moving window:

• For v1.0
  • Replace len magic with count_valid method in ring buffer and moving window #664
  • Remove setitem magic from ring buffer #670
• Post v1.0?
  • Expose gaps in moving window #558

[matthias-wende-frequenz]

FYI @cwasicki @thomas-nicolai-frequenz

[thomas-nicolai-frequenz]

I don't understand why we could not design it in a way where the user passes his numpy array by reference the moving window is writing to. This way we could avoid the need for the fill_gaps parameter. In that case the numpy array could be initialised with whatever values the someone wants.