Node slicing extended

When seeking a section slice, the amended code returns to the start_edge
and stop_edge, to include fractions of an index. This means that values
returned may include interpolated endpoints and hence increase accuracy.
Gear Extension and Retraction and minor track fix

Gear Extension and Retraction periods now take account of gear red
warnings as well as gear down status.
Landing taxi in phase now checks for valid lat/lon data to join up. (In
some cases the localizer was masked at the end of the landing track).
Hide invalid KPV locations

KPVs which are set to times before valid lat/lon data is available are
now not plotted. This normally relates only to accelerometer offsets
which have index=0 and were appearing in mid-Atlantic.
Library refinements.

runway_heading and runway_length now raise exceptions when they cannot
compute a result.
smooth_track_cost_function now uses different weights according to the
sample rate.
Tests for library refinements.