added option to resample png and tiff files
Update source_base.py
added poet base class, updated source base class and added download
functions
Merge branch 'develop' of https://github.com/TUW-GEO/poets into develop
Merge branch 'develop' of https://github.com/IsabellaP/poets into IsabellaP-develop

Conflicts:
	poets/image/resampling.py
Merge branch 'IsabellaP-develop' into develop
Merge branch 'IsabellaP-develop' into develop
updated data sources, added download module
Merge branch 'develop' of https://github.com/TUW-GEO/poets into develop

Conflicts:
	poets/io/ECV/interface.py
updated ECV source class
Merge branch 'develop' of https://github.com/TUW-GEO/poets into develop
formatted code
bugfix