Glueviz

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* Fixed a bug which caused the current slices to be lost when adding a second
dataset to the image viewer. [1581]

* Fixed a bug when two datasets with a different number of dimensions
were in an image viewer and a subset was created. [1577]

* Fixed issues when attempting to close a viewer with the delete key. [1574]

* Disabled default Matplotlib key bindings. [1574]

* Fix compatibility with Matplotlib 2.2. [1566]

* Fix compatibility with some versions of pytest. [1520]

* Fix calculation of dependent_axes to account for cases where there
are some non-zero non-diagonal PC values. Previously any such values
resulted in all axes being returned as dependent axes even though this
isn't necessary. [1552]

* Avoid prompting users multiple times to merge data when dragging
and dropping multiple data files onto glue. [1564]

* Improve error message in PV slicer when _slice_index fails. [1536]

* Fixed a bug that caused an error when trying to save a session that
included an image viewer with an aggregated slice. [1561]

* Fixed a bug that caused an error in the terminal if creating a data
viewer failed properly (with a GUI error message). [1501]

* Fixed a bug that caused performance issues when hiding all image
layers from an image viewer. [1557, 1562]

* Fixed a bug that caused layers to not always be properly removed
when deleting a row from the layer list. [1502]

* Make JSON circular reference errors more explicit. [1529]

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* Improve plugin loading to be less sensitive to exact versions of
installed dependencies for plugins. [1487]

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* Fixed issues with PV slicer and spectrum viewer when changing axes
in the parent image viewer.

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- Fixed __version__ variable which was set to 'undefined'. [344]

- Fixed configuration for tox testing tool. [344]

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- Fixed missing package data.

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- Make it possible to view datasets that are linked but not on the same
pixel grid together. Now also requires datasets to always be linked
in order to be shown in the same viewer. [335, 337]

- Fix compatibility with the latest developer version of glue. [339, 342]

- Fix bug with hidden layers in the 3D scatter viewer becoming visible after
saving and loading session file. [340]