Helper function that checks the number of rows (corresponding e.g to quantiles or samples) per forecast. If the number of quantiles or samples is the same for all forecasts, it returns TRUE and a string with an error message otherwise.
check_number_per_forecast(data, forecast_unit)
Returns TRUE if the check was successful and a string with an error message otherwise.
A data.frame or similar to be checked
Character vector denoting the unit of a single forecast.