File Input

This is typically the first module to appear in a Pablo data analysis
graph.  It specifies a file in the Pablo Self Describing Data Format
(SDDF) to be used as input to the graph.  

During configuration, the user is prompted to indicate which file this
module will use.  The module can read either ASCII or binary SDDF files.
The file used can change from run to run, but all files used by a given
module must have the same record types since all configuration
information in downstream modules is dependent on what the file input reads.

When the graph is run, the file input will read records from the input
file,  verify their format, and pass the records to downstream modules.
A graph can have multiple file inputs, so multiple SDDF files can
contribute to a running graph.
