HDFSFileLocator, & added childLocators method.

Multiple changes:
* The 'childLocators' method has been added to FileLocator.
* Including implementation of childLocators in all FileLocator subclasses.
* added the HDFSFileLocator implementation to read files out of HDFS
* implemented some missing methods in HTTPFileLocator.

(Joint work with Carl Yeksigian.)
[ADAM-463] Added AvroMultiParquetRDD

AvroMultiParquetRDD 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.

(Here, "children" is defined w/r/t the childLocators method
implementation in the corresponding FileLocator.)

(Joint work with Carl Yeksigian.)