Incorrect rounding in subscription handlers leads to data loss

The subscription handlers use the write_interval parameter to round the time index. This sometimes leads to duplicated indices which means that values in the subscription handler will be overwritten. Correct behaviour should be that all data points are kept, even if rounding would lead to equal indices.

Additionally, there is a duplicate meaning of the parameter write_interval in the CSVSubHandler. Besides rounding the index values, the parameter is used to determine the buffer size. A new parameter should be introduced to determine buffer size independently.