Clean up properties before parsing

[Faterou]

Originally reported by: Sylvain Hallé (Bitbucket: sylvainhalle, GitHub: sylvainhalle)

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Parsing fails when a formula has a pair of well-balanced, but superfluous parentheses. Example: left-hand side of

((blabla)) And (blabla)

Instead of complexifying the grammar to take this into account, we could easily code a simple cleanup method that fixes parentheses before sending the string to the interpreter.

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• Bitbucket: https://bitbucket.org/sylvainhalle/cornipickle/issue/22