Warn for non-unique model-scenario index when adding timeseries

[danielhuppmann]

The function add_timeseries() ignores the columns model and scenario when data is added to a scenario. This just led me to inadvertently upload a dataframe with data from multiple scenarios, where just the latest data for each variable was then saved to the database.

We should add a warning (or exception) when a user tries to add data with model/scenario columns that do not have unique values to guard against this mistake.