Ztv

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- bug fix in LoupeImagePanel after redisplay for cmap/clim/etc change
- added add_text/remove_text methods similar to patches for external code to plot on PrimaryImagePanel

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- updated shift-key behavior in slice plot mode on primary image panel when releasing button to avoid having the selected slice 'jump'
- bug fixes
- fixed MAJOR bug of a memory leak related to matplotlib; ztv hadn't been properly calling cla() to clear axes and thus matplotlib objects were just piling up in memory. (There may be some remaining similar leaks to be fixed, but the vast majority of the culprit was the imshow()'s and those are now fixed.)

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- implemented cursor readout in plot_panel
- implemented shift-key for locking to horizontal/vertical plot slices on primary image panel
- fixed some logic in cursor modes when entering/exiting primary image panel with button pressed

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- added viridis colormap
- couple of API bug fixes (thanks samverstocken!)

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- fixed bug that wx timestamps were sometimes large negative unchanging integers, which killed click-and-drag zooming

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- cleaned up release of current codebase incorporating lots of bugfixes from recent months.

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