Different API for passing args and kwargs

[rmjarvis]

I was adjusting TreeCorr to use this package rather than my original file, and I discovered that I had already added the ability to pass args, but with a slightly different syntax. Specifically, you just pass the args or kwargs as additional parameters to mpi_exec, rather than explicitly packaging them in a list or dict. I think this is a bit cleaner.

[joezuntz]

Makes sense unless there's any additional arguments of any kind we could imagine adding to mock_mpiexec later, since then backward compatibility would break. But I can't think of any off-hand that wouldn't be better served by a separate function.