PFM: change write algo

Measurements were performed on a series of export of 100 images for each write algorithm.
Images was exported to ST3000DM001-9YN166 using this dt-cli:
```
for i in `seq 1 100`
do
 /usr/local/bin/darktable-cli 5DM30001.CR2 5DM30001.CR2.xmp darktable_exported/5DM30001.pfm
done
```
Image history stack was empty - all modules that could be disabled was disabled.
Current implementation seems to be worst, and proposed seems to be best.

Spreadsheet with performance measurement results: [link](https://docs.google.com/spreadsheets/d/1h188xaHnrlR64_m_mlFKl_6ihzf7X0muK313SRihPnw/edit?usp=sharing)
SQL dump of measurements: [link](https://drive.google.com/file/d/0B0f1NIO0-n9AZGN6dlI3bHZIc00/edit?usp=sharing)
Query: 
```
SELECT `id`,`algo`,CONCAT(`meas_name`,' ',`measurement`) AS name,MIN(`value`),MAX(`value`),AVG(`value`),STD(`value`) FROM `data` GROUP BY `algo`,`name` ORDER BY `name` ASC
```