types and parsers for epoch time and absolute DateTime

At least measured by loc, it's a lot simpler returning just an Option: https://github.com/stucchio/timeserieszen/blob/reader2/src/main/scala/timeserieszen/retrieval/AtTime.scala#L119

Also note the distinction between monoidal `|+|` and the simple `Plus` typeclass version `<+>` (not that I think it matters in the case you used it, just a general FYI which bit me once or twice):

```
Some(Some(1)) <+> Some(Some(2)) == Some(Some(1))
Some(Some(1)) |+| Some(Some(2)) == Some(Some(3))
```