library: Added threshold kwarg to slices_from_to

Threshold can be used to stop slices_from_to creating thousands of
slices from noisy signals.
flight_phase: Corrected approach docstrings

Brief description of changed behaviour due to find_low_alts
library: Change slices_from_to threshold to 0.1

Changed slices_from_to threshold to 0.1 to fix tests and provide more
leeway. Updated tests, added comments.
Merge branch 'master' of https://github.com/FlightDataServices/FlightDataAnalyzer