Imexam

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- updated classifiers in pypi
- cleanup of branch

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- moved extension local to imexam
- cleanup of travis tests and general package reorg
- renamed xpa extension for compatibility and updated to version 2.1.19
- added option for center of mass to aperature photometry centering
- update aper_phot to return tuple of photometry information without plotting
- optional error array can be sent to aper_phot when using without plotting
- updated example jupyter notebooks to be compatible with current functionality
- changed the ext_build process a little and added a flag to skip remaking the c code
- remove leftover iraf variable names
- removed support for the grab function under Darwin
- add deleted import of xpa back to utils (192)
- adapt for change to photutils API for aperture areas (193)
- fix ginga display of HDUList
- add check for ds9 in alias as well as well as path
- python2->3 class and printing updates
- removed dependence on astropy_helpers

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** THIS WILL BE THE LAST VERSION THAT SUPPORT Python 2.7 **

- travis and appveyor testing updates
- radial profile plot centering fixed to more correctly calculate the fractional center offsets
- cumulative radial profile flux calculation should now be correct
- the fit_gauss_1d function call was changed to accept the radius and flux array so that they
could be constructed correctly for multiple circumstances
- documentation updated and new simple walkthrough added
- MEF fits images with IMAGE arrays in the primary HDU should be detected correctly now
- now possible to give load_fits an in-memory fits object
- code cleanup and minor bug fixes
- background fit added to 1D and 2D Gaussian fits
- plotting AiryDisk2D fit is now possible
- unit test updates
- new options added to the aperture phot parameter set that allow users to plot the used apertures
- ZScaleInterval added from astropy.virtualization to set the color range on the data for aperture photometry plot
- replaced the sigma to fwhm lambda with the astropy constant for conversion
- added cursor move recognition using the arrow keys during the imexam loop, however, depending how the user has their windowing focus set, the DS9 window may loose focus, forcing them to move the cursor manually back to the window. Cursor moves are only implemented for DS9, not Ginga.

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- fixed show_xpa_commands bug sending None instead of empty string
to the xpa library
- fixed logic of connect method. When a target is given, do not look
for an executable
- view method now supports loading cubes when there are 3 dimension in the given array
- logic bug in ds9 class init updated to warn when user specified target doesn't exist

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- fixed xpa bug holdout from updating for windows specific code
- changed default connection type from local to inet when XPA_METHOD not specified in users environment

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- fixed a text error in the display_help() so that now the correct version loads the documentation
- Windows users can now install from source. The setup will ignore the cython and xpa necessary to build the DS9 interaction, and users will only be able to use the Ginga HTML5 window, they can also use the Imexamine() functions without any graphical interface.
- Documentation updates, mostly specific information for Windows users
- Added python 3.6 to the test matrix as well as apveyor for the windows build
- Updated XPA module to v2.1.18
- Made fits checker smarter to deal with older simple fits files where EXTEND is true but there are no extensions
- fixed bug in fits loader for ds9 multi-extension FITS files, made load_fits() prefer the extension specified in the key rather than the image name