info: track start/end ele/temp better

quite often first/last chunk in recordings don't hold proper values.

This diff makes the info filter find/remember the first/last valid value
for temp and ele.
info: ascent/descent calculation, remove incline

"incline" was misnamed and is officially deprecated (but still working).

Instead please use "ascent". descent is now calculated, too.
Pwx: copy with bad timestamps

if delta of subsequent timestamps was too small, files failed to open due
to consistency checks.

Instead we're now ignoring chunks when no time elapsed.