warning for MERGE and ACCUMULATE

When doing a full merge (or any merge that inserts new timestamps) might produce surprising behavior for the end user: if merging 1h precipitation data with 10 minutes meteo data, the re-accumulate resampling won't properly re-accumulate the data as it will find the hourly values proceeded by 5 nodata values and therefore assume that it is 10 minutes data with lots of gaps... We could at least print a warning on the screen when MERGE and ACCUMULATE are used together to warn of this possible side effect.