fixed wrong type conversion for numeric variables.

Created originally on Bitbucket by tonioo (Antoine Nguyen)

Was already merged in Bitbucket before import, marked as merged by the import user

Currently when generating ODS files, variables containing numbers (int or float) are automatically converted to strings, which is not a good idea if you try to use mathematical functions on the generated cells. This PR tries to fix this behavior by checking variable type at rendering time and use float instead of string when needed.