Allow to write simple mathematic formulas in Integer / Float fields
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Functionnalities:
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    * Possibility to tip a text like "=45 + 4/3 - 5 * (2 +1)";
    * if the formula is correct, The result will be computed and displayed;
    * if the formula is not correct, the initial text is displayed;

Documentations:
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    * Video: http://www.youtube.com/watch?v=jQGdD34WYrA&hd=1

Technical informations:
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    * Overloads "instance.web.form.FieldFloat"; (so works for fields.integer &
      fields.float);
    * To compute, the module simply use the eval() javascript function;
    * Rounding computation is not done by this module (The module has the same
      behaviour if the user tips "=1/3" or if he tips "0.33[...]");
    * avoid code injonction by regexpr test: "=alert('security')" is not valid;

Limits:
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    * Only supports the four operators: "+" "-" "*" "/" and parenthesis;

Copyright and Licence:
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    * 2013, Groupement Régional Alimentaire de Proximité
      (http://www.grap.coop/)
    * Licence: AGPL-3 (http://www.gnu.org/licenses/)

Contacts :
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    * Sylvain LE GAL (https://twitter.com/legalsylvain);
    * <informatique@grap.coop> for any help or question about this module.