Convert all File stuff to Path; add fallback to Config from DistConfig when required

[nilesh-c]

This was done to convert all remaining File-specific stuff to using Path. That is necessary because we have to support Hadoop file systems (HDFS etc.) equally well - already discussed.

As for this particular change, I personally feel this is getting pretty ugly because the client code is having to juggle between DistConfig and Config when it comes to dealing with mappingsDir, 'ontologyFileanddumpDir`. But this is supposed to be handled behind-the-scenes by the config object.

I propose to reject this PR and combine Config and DistConfig into one single DistConfig. I'll make a new PR. Here's how DistConfig will look like:

/**
 * Class for distributed configuration
 */
class DistConfig(configProps: Properties, config: Config)
{
    // add all Spark and Hadoop-specific stuff

   // dumpDir, mappingsDir, ontologyFile => do the fallback to config logic here and delegate to config accordingly.

   // for the rest of the config.* methods, simply delegate. Act as wrapper.

}

[nilesh-c]

@jimkont @jcsahnwaldt please discuss how you think we should proceed. @jcsahnwaldt had mentioned DistConfig extending Config, using generics etc. But I can't see how we can use inheritance here because dumpDir, mappingsDir etc. are not remaining Files anymore - they are Option[Path]s now in DistConfig. :(

I suggested the delegation/wrapper approach, but I'm open to ideas.