check if using tdim="timedimension" fails in detect() function

[paolap]

A user had an example of detect function failing when a time dimension with name different from 'time' is passed even if dim is correctly set to time dimension name.
Failure showed in agg_df as data frame doesn't have the time column.
I have to add a test for this and fix if necessary. Bug could have been introduced in most recent version