Adds the ability to save and restore a greyscale stretch to/from a Cube (#3717)

* Update meta.yaml to rename conda package to isis from isis3

* Initial stretch attempt

* Now will write a single stretch with a name and type to the cube

* Stretch updated to inherit from Blob, re-loading a saved stretch works but only if Linear right now

* Clean up Stretch class

* Completely move stretchTypes into stretch class, and some minor cleanup

* Fixed combo-box not updating bug and more cleanup

* Further cleanup of propagated earlier changes with types and names being removed from unnecessary classes

* Removed added unneeded member variables from StretchType

* Cleanup StretchTool class

* Update enter-text dialogs to drop down selection options

* Wrapped some long strings

* Initial commit to address most of review comments

* Removed buttons in case of RGB stretch and add a 'Color' PvlKeyword to Stretch output

Co-authored-by: Stuart Sides <ssides@usgs.gov>