Skip pan/zoom tool registration
Add warning when removing
Decide based on x and y range, not axes (which are optional)
remove objects as well
Revert "remove objects as well"

Reverts commit 68ef9faa4c592b3721387851b580470eec7e5f9d

(Will need to discuss the tool registration process further)
Merge pull request #727 from ContinuumIO/fix/categorical_tool_removal

Skip pan/zoom tool registration