WIP: Spark catalog

Created AttributeCatalog, so that any RootJsonReadable type can be read to and from the catalog by layer ID and attribute name. Still trying to figure out how to work that into the catalog infrastructure, tried out a couple API's but didn't really find something that worked yet.

Also added the ability to read a single tile out of AccumuloCatalog (this needs to be lifted up to the Catalog trait and implemented for HDFS).