GEOMESA-173 Extract avro features into its own subproject

Moved AvroSimpleFeature into its own subproject.  Implemented
AvroSimpleFeatureFactory to create avro simplefeatures when
using SimpleFeatureBuilder
DensityIterator.scala uses the code

    AvroSimpleFeatureFactory.featureBuilder(projectedSFT)

Seems to be doing the same thing. Should we replace these lines with that as well to have a single block of code dealing with the factory/hints way of creating featureBuilders?