merging and redo

### new functionality:
* merge before remove primers. this obviates the intersection step, so that script has been made a utility.
* `remove_primers.py` now removes forward and reverse primers
* some new support for zcluster (still tentative)

### new options:
* `--redo` will rerun otu_caller with the previously used commands *for that directory*. the commands are stored in `.SmileTrain.commands.pkl`, a pickled namespace.
* `--n_cpus` is now called `n_split`. the short name is still `-n`.