Helios E6 (quiet-time) single detector count rates

The new Helios E6 quiet-time dataset provides full-mission time series for Helios 1 and Helios 2 based on single-detector count rates used in the SPEARHEAD Deliverable D5.3. Each record is converted to calendar datetime format and contains the midpoint time, start and end times, accumulation time, the single-detector count rates D2, D5, CE, and AN with their statistical errors, the derived ratio D2/D5, and a quiet-time flag. The flag `quiet_time` is set to 1 when the record has sufficient accumulation time (`acc > 0.2` hours) and its D2/D5 ratio lies within the Gaussian quiet-time band defined by `mean ± 4 sigma`; otherwise it is set to 0. Thus, the dataset preserves the complete mission data while making quiet-time intervals directly identifiable for further analysis.