Get the current batch, rep, and overall index

[wlandau]

In tar_rep(), tar_map_rep(), etc., it is sometimes useful to know the batch number, rep number, and overall index number (batch * (batch_size - 1) + rep). For example, sometimes a user may want to set a custom seed this way. Implementation for tar_rep() and tar_map_rep() is straightforward, I'll need to look back at the other target factories that may also benefit.