Make the directory/file vals lazy so that they don't throw initializatio...

These need to be lazy or else things like if (! dir.exists) throw error("dir "+dir+" does not exist") or File initialization errors in `Config` can mess with the initialization of `DistConfig` (which prevents distributed extraction from starting).

This mainly occurs if a path like hdfs:///somepath/somefile is given to base-dir in spark-config.properties, and the base-dir in the general config.properties is omitted, or set to hdfs:///somepath/somefile.

The PR for the Config side of things is at https://github.com/dbpedia/extraction-framework/pull/252