fix PWX export: handle end of gaps gracefully.

As I got it pwx by itself has no concept of recording intervals. "smart
recording" and other strange data require variable recoring intervals.

On the other hand it's not recording the duration of a sample explicitly.
This means the duration needs to be derived from the previous samples
timestamp (assuming the sample timestamps are referring to the end of the
sampled period).

This diff writes an empty timestamp at the end of each gap so that the
next real sample/ridepoint can calculate the correct duration.

Please note that this may unhide some deficiencies in GCs file reading in
exports.
fix PWX export: always write summarydata

xml schema is requiring summarydata to exist. Otherwise upload to
trainingpeaks fails.

Block was omitted when no intervals were defined.
PWX export: keep timestamp + distance precision

Pwx does support for arbitrary sample lengths. No need to chop off
timestamp precision and introduce bad data + jitter.

Instead we should look at fixing data import and consider resampling on
read.

Distance is defined as double, aswell - so, let's keep the precision,
aswell.
PWX: read/write temperature

added temperature to read/write
Merge pull request #682 from rclasen/fix-pwx

several pwx fixes / improvements