Support for GTF files: GtfCodec and GtfWriter

[lindenb]

Description

This PR adds a support for GTF files.

The classes Gff3BaseData, Gff3FeatureImpl, Gff3Feature are used for the GTFCodec. I didn't want to write a class 'GtfFeature' as the classes would be identical.

AbstractGxxCodec is now the base class for Gff3Codec and GtfCodec.

AbstractGxxConstants is now the base class for Gff3Constants and GtfConstants.

AbstractGxxWriter is now the base class for Gff3Writer and GtfWriter

GtfCodec builds a tree using the attributes gene_id and transcript_id.

Test for GtfCodecTest and GtfWriterTest have been added.

The code was formatted in eclipse using "java-style-eclipse.xml"

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