precip_mm_diff for first interpolated sample could be non zero

Closes #50 (closed)

Added code to datastore:get_station_data() to return the last piece of data before the requested starttime plus all data within requested time range. process_station() will then calculate the correct value for precip_mm_diff at the requested starttime. The dataframe is then trimmed down to original requested size in process_fire(). If the requested starttime is None or the beginning of the dataset, then precip_mm_diff will be zero.