Warn users when applying "not" to a continuous variable

[pauljensen]

Warn users when they apply the "not" operator to a continuous variable to avoid un-intuitive results (see issue #23 for example).

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• Bitbucket: https://bitbucket.org/csbl/tiger/issue/28
• Originally reported by: Paul Jensen
• Originally created at: 2012-08-10T21:16:07.743