Code improvements in Scala reference implementation

[dbehrouz]

• Use of more compact code
• Use immutable objects in TF IDF computation
• Fixed a bug in TF-IDF where the supplied input to the program was being ignored and a static location was being used as input.

[rmetzger]

Thanks a lot for the pull request.
@fhueske has the full overview over the Flink training material. He just left for a one week vacation, so you'll have to wait until then ;)

But from my point of view, the changes look reasonable.

[mxm]

Thanks @bderak for your pull request. I agree with the Scala immutable object changes and the bug fixing in TF-IDF. However, it was intended for the code to not be very compact in order to make novices familiar with the general Flink transformations. It's not very Scala-esk I agree :)

[fhueske]

Hi @bderak, thanks for the fixes! As @mxm said, we used groupReduce instead of sum on purpose to teach the concept of reduce. The other changes look very good.
Thanks, Fabian

[dbehrouz]

great.
Sorry I was travelling as well these past weeks. I ll make the changes soon ...