rtefcst reloaded (almost) complete

* Re-introduced BBox, Position, eliminated LatLon in rtefcst, all times
now np.datetime64 or np.timedelta64 (incl. tests), replaced string
constants by conv where possible.
* Works with new windbreaker fcst / xray.
* Pulls all available (non coordinate) forecast variables and writes
non-wind data into WP description.
* Moved logic for Wind.readable into new NautAnge method (Wind interface
unchanged).
* Internal unit handling now consistent in rtefcst: everything converted
to default units upon acquisition and reconverted for external output if
required.
* Added _angle to units with default 'degrees' and conversion to/from
'radians', added hPa to available pressure units.
* Added queryDict to Route, move compass_dir() from units under
NautAngle to avoid circular imports.
* Added WIND to conventions.
* Some new unit test in test_rtefcst.py, but RouteForecast tests still
disabled (need to fix expected results).