Add timezones to rainfall data

Relates to #42 (closed)

• add station timezone, name (and other columns) to stations.csv

• update mesowest timeformat to include timezone offset, still use local response time if possible

• update pandas date handling to store timezone offset

• verify update_data last updated timestamp is sent to mesowest using UTC

• bonus: filter list of stations to those that have precipitation data

  this would simplify code to not have to check whether "precip_intervals_set_1d" exists, since all stations being used would produce this output