Rewrite png_generation script to use numpy and gdal instead of the nens.asc libraries.
Improve error handling of colormap.
Change meaning of leftbounds in colormap, they are now inclusive.
Make the calculation of opacity faster.
Reorganize tests.
Let the png_generation task use a custom tmp_dir.

Reinstate the "__revision__" bit in png_generation, apparently it's still used.
Close files before trying to delete them (for Windows)
WIP
Fix generation of .pgw files
Change import of scipy.misc.
Remove unused script.