fixes to shef exporter

[Enovotny]

Summary

Fixes for issue #73, #74, #75 plus
related robustness improvements to the CDA exporter/loader and the export CLI.

Changes

CDA exporter (shef/exporters/cda_exporter.py)

• Skip time series whose CDA fetch errors, returns a non-dict payload, or has
  no values in the window — log and continue instead of aborting the whole
  export.
• Build the JSON payload via json.dumps(list) rather than hand-assembling
  [...] with comma bookkeeping.
• Defer make_export_transforms() until needed, and pass the specific group_id
  when exporting a group.

CDA loader (shef/loaders/cda_loader.py)

• make_export_transforms() now accepts an optional group_id so a single group
  can be fetched instead of all SHEF Export groups. Tracks loaded groups to
  avoid redundant CDA calls.
• Skip assigned time series with missing/empty alias-id (root cause of shefParser export command generates empty SHEF file when some aliases are empty #73)
  with a warning instead of producing an empty SHEF file.
• Added group_office_id filter on cwms.get_timeseries_groups.

Export CLI (shef/shef_parser.py)

• Replaced the hec.HecTime dependency in export with a local parse_dt
  supporting ISO 8601 and HEC-style relative times (T, T-1D, T+2H, …).
• Require both --start-time and --end-time; raise click.BadParameter on
  unparseable input.
• Remainder of the file is whitespace/line-wrap reformatting (no logic
  changes).

Optional dependencies (pyproject.toml, dss_loader.py, abstract_exporter.py)

• cwms-python and hecdss are now optional extras (pip install .[cda], .[dss]).
• dss_loader guards imports and raises a clear ImportError if hecdss is
  missing.
• Removed unused top-level import cwms from abstract_exporter.py.

Tests

Three new test files:

• tests/test_cda_exporter_empty_series.py — regression for shefParser export command generates empty SHEF file when some aliases are empty #73; export
  succeeds when some series in a group are empty or error.
• tests/test_cda_loader_make_export_transforms.py — covers single-group
  loading, alias-id skipping, and caching.
• tests/test_export_time_window.py — covers ISO and relative time parsing for
  the export CLI.

Test plan

• pytest tests/test_cda_exporter_empty_series.py
  tests/test_cda_loader_make_export_transforms.py
  tests/test_export_time_window.py
• Run shef export against a real CDA office with at least one group containing
  an empty-alias entry and verify the output SHEF file is non-empty.
• Verify pip install . (no extras) still imports shef.shef_parser without
  hecdss/cwms installed.