Add ability to load data sources without providing a path

Previously, to load a JSON file with the data in a separate bp file, you had to specify the directory or path of the data file in the "paths" map provided to the DataSetReader read methods. This is a problem because it makes the file formant not self-descriptive. When loading data in an application like ParaView, you have to type in the directory to the bp file attached to a magic string defined somewhere in the JSON file. Every. Damn. Time.

This change allows you to specify a relative path to the data source .bp file within the JSON file. If you do not specify a matching path, the reader will look for the .bp file relative to the JSON file. If you do provide a path, then that path is used. Thus, the old behavior is preserved.