Sdss-coordio

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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()``

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* Implement ``zbplus2`` FVC transform.
* Handle Hogg coefficients by loading from the first ``wokcalib`` dir on path.

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Several changes to ``extraction.extract_marginal()``:

* Fix a potential problem in plot box.
* Deal with no detections in ``extract_marginal()``.
* Use gray colourmap in ``extract_marginal()`` plotting.

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* Make ``seaborn`` a dependency since it's needed by some of the extraction routines.

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* No major changes since 1.5.0b1. Tagging as full release.

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* Added additional tools for extraction and fitting the marginal distribution.
* `17 <https://github.com/sdss/coordio/pull/17>`__: implementation of the offset function.