Niondata

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- Change shift/align functions to use spline-1st-order; add Fourier variants as alternative.
- Fix calibration bug in xdata concatenate (and some cases of hstack, vstack).
- Add function to generate elliptical masks.
- Change FFT to put calibration origin at 0.5, 0.5 pixels from center.

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- Improve handling of squeeze/calibration for sequence measurements.
- Add new navigation properties (combo of is_sequence and collection) to data.
- Support slicing on RGB sequences (for display data).

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- Added optional registration area bounds to align and register functions.

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- Added mean function. Add keepdim param to mean/sum. Allow negative indices.

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- Fix display RGB calculation on integer images.
- Add methods for better control of data ref count.

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- Add setters for timezone, timezone_offset, and timestamp.