Sdss-coordio

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* Update offset functions to include following functionalities:
* New defaults for offset function parameters
* Observatory dependent offsetting as default
* Offsets are calculated using all magnitudes for a source and the maximum is used as the returned offset

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* Also calculate measured RMS using only cameras that were used for the fit.

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* Calculate global fit RMS using only cameras that were used for the fit.

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* Update default ``safety_factor`` for bright and dark time.
* Calculate guider fit RMS.

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* Adjust nudge model for new FVC frame sizes at APO and LCO.
* Added binding for the SOFA ``iauRefco`` function.
* Allow to call ``FocalPlane()`` with ``use_closest_wavelength`` and an arbitrary wavelength to use the ZEMAX model with the closest wavelength.

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* In ``get_marginal()`` allow to normalise the distribution using an input value. Otherwise use a narrow regions around the centre of the box.
* Several improvements to Gaussian fitting in ``fit_gaussian_to_marginal()``. Mark marginal fit as bad if the fit is unsuccessful.
* Improve plotting of detections in ``extract_marginal()``
* Sort detections by flux and allow to cap the number of sources returned.
* Require that ``tnpix`` be larger than ``minarea`` in ``sextractor_quick()``