DebugReplay: each 3d fix has a valid gps altitude

Don't try to guess it's availability based on the height value, but only
on the validity flag of the B record.

The IGC file spec says at A2.4 Units: GNSS altitude drop-outs should
be logged as zero altitude. This is obviously only for convenience, as
A4.1 states that 2d fixes (no GPS altitude) should be flagged with a 'V'.
The latter should always have precedence over the zeroed altitude value
and therefore it should be safe to assume a valid gps altitude for
all 3d fixes.
DebugReplay: don't set the baro altitude
DebugReplay: always set pressure altitude

Pressure altitude is always available from IGC files, see A4.1.2,
Fix validity: "pressure altitude data must continue to be recorded
using UTC times from the RTC".
PythonConverters: set gps_valid flag only if gps alt is available

Be consinstent with IGCFix::Apply()
PythonConverters: fall back to gps_altitude if no pressure_altitude available

Be consistent with IGCFix::Apply()