Added route forecasting logic in sl/lib/rtefcst.py.

Test module in test/test_rtefcst.py, with test data
/test/rtefcst_test.csv (route data) and /test/rtefcst_test.fcst
(windbreaker packed forecast file covering route).
Small updates to sl/lib/objects.py.

See "Notes to Alex" in comments.
added handle_rtefcst and re-shuffled argparse subparser setup
Added files directory with windarrow waypoint icons for OpenCPN.