GFF loading should be able to specify feature types to be ignored

[adf-ncgr]

lis-bio-sources/lis-annotation/src/main/java/org/intermine/bio/dataconversion/AnnotationFileConverter.java throws an Exception if it meets a feature that hasn't been mapped to an Intermine class. Some files contain legitimate features such as introns/start_codons/stop_codons whose utility being represented in the mines is dubious. We could just opt to purge them from the datastore files, but it does seem as though this is one of those things the loading process ought to be able to provide some freedom to the mine operator to gracefully ignore things not worth bothering with. This came up when loading Medicago/truncatula/annotations/HM078.gnm1.ann1.3RNP. For now I'll just make a local copy of the file and exclude them.