Integrate new batch generator that combines foreground and background loci

[zahoorz]

----- for the generator

1. The input json will have new field for background loci
2. The user will specify #samples for foreground and background (-1 for all)
3. The background_only option will dictate whether the bed file in the background_loci field of the input json will have weight as zero or one for mnll loss (e.g. in case of training background model the weight for background_loci will be 1)

---- for integration

1. Add command line argument to pass background_only flag to the batch generator