return solar time with get_ts

i want to add this new item to streammetabolizer:

```
* BEHAVIOR CHANGE: get_ts now returns a new column, sitetime, that contains the
date-time in in solar mean time. DateTime still does, and always has, returned
the date-time in UTC time. sitetime also has tz()=='UTC', but it's much closer
to the local non-daylight-savings clock time.
```

but need to think about whether this is the best approach

(was mis-posted under https://github.com/USGS-R/streamMetabolizer/issues/179)