Changes in GFF3 export

This PR introduces some changes in how GFF3 output is created from the internal Artemis feature representation. In particular, I have added means to handle how qualifiers map to attributes, and  how their values are formatted in the GFF output. This includes a generic mechanism to map/clone/ignore qualifiers, and to associate mapped attributes with pieces of code describing how to transfer the contents of the corresponding qualifier StringVector into a properly escaped, potentially multi-value GFF3 attribute. This mechanism is used to implement some conversion conventions discussed with TriTrypDB.