[ADAM-463] AvroMultiParquetRDD loads multiple Parquet files

[ADAM-463] Added AvroMultiParquetRDD

First, updated the FileLocator to include a childLocators method, and added its implementation in all the child FileLocator implementations.

Next, added an HDFSFileLocator implementation. 

Finally, added AvroMultiParquetRDD, which takes a FileLocator which indicates the _parent_ directory (in HDFS, or S3) of a set of parquet files, and which loads _all_ row groups from all the parquet files that are immediate children of that FileLocator.
        
(Joint work with Carl Yeksigian.)

As an aside, what's the use case for HTTP access in the end? Not really germane to this PR, but should be documented eventually.