GEOMESA-138 encode avrosimplefeature field name

included tests
it was a suggestion by jahhulbert, I think I interpreted it correctly. basically it would prevent duplicate encode and decode calls, no idea what benefit if any this provides. At the moment it does do both directions, like (a, v), (v, a) but only if both the encode and decode functions have been called. you could just do it in one go at the encode stage and make the decodeAttributeName function just be a map get.