* Fixed issue #37

 (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.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>