Pvectorize

Seeing that @mllg implemented `bpmapply`, I decided to take another crack at implementing a `bpvectorize` that can parallelize multiple vector arguments. First, I did a bit of cleanup (81b043b & e880dc1) and preparation (8dae81c, though this one isn't actually used), then I implemented `bpmvec`, which is like `bpvec` only it can vectorize multiple args. Then, using that as a base, I implemented a new `bpvectorize` which takes a character vector `VECTOR.ARGS` that tells it which arguments are vectorized arguments to be parallelized. Lastly, I added in tests for `bpvectorize` and docs for both `bpvectorize` and `bpmvec`. The bvectorize tests also implicitly test bpmvec, but it would probably be a good idea to add tests directly for bpmvec as well.''