GlideComputerBlackboard: remove empty constructor
GlideRatioComputer: eliminate the "last_basic" parameter
Merge branch 'master' of git://github.com/hjr/XCSoar
GlideComputerAirData: eliminate "last_basic" parameter from FlightTimes()

Simplify the warp check.
GlideComputerAirData: remove obsolete prototype Average30s()
WarningComputer: eliminate the last_basic parameter

Use a DeltaTime object to keep track of times, also replacing the
GPSClock instance.
TaskComputer: eliminate the last_basic parameter

Was only used for a time advance check.  Use location_available
instead, compare with last value.
GlideComputer: use DeltaTime for trace_history updates
GlideComputerBlackboard: remove last_gps_info

Unused.
test/RunTask: replace last_basic/last_calculated with last_aircraft_state
BasicComputer: move code to GroundSpeedComputer
GroundSpeedComputer: use DeltaTime

Eliminate the last_basic parameter.
BasicComputer: always call ComputeAirspeed()

Don't check NMEAInfo::HasTimeAdvancedSince().  This method can now be
removed.
GlideComputer: call ProcessAutoTask() unconditionally

Remove the time_retreated() check, because it's useless: right after a
time warp, take-off will never be detected.
GlideComputer: call ProcessClimbEvents() unconditionally

See last commit.
GlideComputerBlackboard: remove time_retreated()

Unused.
GlideComputer: remove virtual destructor

Obsolete.
GPSClock: remove unused method DeltaAdvance()

Most callers have moved on to class DeltaTime.
DeltaTime: un-inline method Update()

This method has grown so complex, it should not be inlined.
*: use std::fill_n() instead of std::fill() where appropriate