fix number formating (nolap report editor #123)

https://github.com/28msec/nolap-report-editor/issues/123
The formating doesn't change for Decimals <= 0 (nevertheless, in that case formatNumber is still used in the return clause). For example:
123,123.12 (Decimals 2)
123,123 (Decimals 0)
123,000 (Decimals -3)