Ignore negative values

[tetienne]

Fix #22

Sometimes Veolia returns invalid values, like a huge number. They fix this value sending just after a huge negative number. At the end the error is resolved, and does not appear anymore.

date;consommation(litre)
01/03/2018;275
02/03/2018;154
03/03/2018;379
03/03/2018;-1754359
04/03/2018;1754558

[github-actions]

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