Allow streams files without a time axis

Some streams files are intended to be static, but that's seemingly not allowed. Instead, every streams file (and, presumably, every variable being read from streams files) must have a time axis. This requires the addition of a "dummy" time axis on files that don't need it.

I think it would be better if a time axis were optional. This would make the code cleaner (avoiding time-related stuff that's not needed), avoid headaches (making an input file but forgetting to add the dummy time axis), and improve strictness (throw an error if a user supplies a streams file with a time axis for an input that's supposed to be static).

As far as what this would look like, I'd think the following arguments to `shr_strdata_init_from_inline()` would be made optional:
- `stream_yearFirst`
- `stream_yearLast`
- `stream_yearAlign`
- `stream_offset`
- `stream_taxmode`
- `stream_dtlimit`
- `stream_tintalgo`

If any of these were provided, then the streams file would require a time axis, and all those arguments would need to be provided.

If those arguments weren't provided but the user-supplied streams file does have a time axis, CDEPS would throw an error.