add stub for exporting survey points csv
add generation of tide files from a USGS gauge
tide data is a precursor to export survey points and needs to be easily editable
calculate and write out survey points to csv file
format exported survey points with fixed-width columns as in old format
rm unused return
use interpolated lat/long values instead of repeated duplicates
depends on https://github.com/twdb/sdi/pull/28
fix: handle cases where trace timestamps are not unique
looks like this can happen if traces on separate frequencies occur within the
same instrument clock cycle
misc cleanup
Merge pull request #75 from twdb/export-survey-points

Export survey points