Bugfix/fix no dem exception
- Fixed issue #37 (closed) (Empty FileNotFoundError in case no DEM is provided by the user). If no DEM is provided by the user, an average elevation (default=0) is used instead. Added corresponding warning and log messages.
- Fixed exception in case an average elevation is passed.
- Fixed a bug that caused 0 or 1 integers provided in the config parameters to be converted to booleans.
- User-provided file paths are now validated directly on startup.
- DEM_Processor instance can now be created from an average elevation (returns a flat DEM).
- RPC geolayer generators are now also compatible with an integer as input elevation.