Added Indexes on FeatureList

[CarmeloFoti]

This patch modifies the GFF-related classes. Added a new method to the class GFF3Reader:

public static FeatureList read(String filename, List<String> indexes)

this method will create an index for each attribute present in the List indexes. In this way, a massive improvement in performance will occur.

In addition, the String.split() method is now being used during the Feature creation (as it was suggested in the code comments).