Created AstromWorkload class which gets created by a task and passed
 to the model.  Currently the model is only using the first astrom data
in the workload.  In the future it will use all of them.
It is now possible to traverse all astrom data in the workload.
Processing candidates still writes to just one file though.
Renamed test files in test astrom data directory so they won't class
 and set up gitignore to make sure output files generated aren't 
committed.
Moved creation and management of writers to the models.  The 
controller can then get the appropriate writer from its model when 
needed.  For candidates processing there are now multiple writers 
creating an output .reals.astrom file corresponding to the .cands.astrom
file that the current source is from.
Updated gitignore in test/data directory with files that are being 
generated by unit tests now that the model creates the writers.