Delete var. from Group when deleting from Manager

Fixes #59.

Added a method to VariableGroup that deletes a variable (if found).  Then
called this from inside VariableDelete.vi to purge VariableGroups of the
deleted variable.
Correct editing variable behavior

Address the issue #59.  This commit should have all desired behaviors
when editing and deleting variable.
Merge branch 'Fix-Variable-Group'