Rename spatialize number_of_output_files parameter

The parameter was number_of_output_files in the documentation, but
number_of_files in the code. Changed to the more precise
number_of_output_files.
Fix vaporize not working with DEM

When a DEM instead of a constant value was being passed as the
"elevation" parameter in the configuration file of vaporize, it was
raising an exception. This was fixed.