rtefcst now uses units for unit conversions/defaults

Adds some conversion utilities to units.py to handle scalars, changes
conversions in rtefcst to use units.py for all conersions. Internally,
wind speeds, boat speeds, etc will be stroed in whatever is defined as
default in units.py
Merge branch 'route-forecast-units' of https://github.com/ms8r/slocum into ms8r-route-forecast-units
Merge branch 'ms8r-route-forecast-units' into route-forecast

Conflicts:
	python/sl/lib/rtefcst.py
	python/sl/lib/units.py