Implement a "merge dataset"

Use this for combining e.g. features
Let's make this only 1 format, so arg changes to `fmt` and all of `self.datasets` are passed the same `fmt`, which is required to be 1 of `mdtraj` or `hdf5`. And since only NumpyDirDataset and HDF5Dataset are supported, we don't need to support any extra kwargs, so we can get rid of that too. I'd rather start small and expand the scope/functionality later than do the opposite. We can always add more later, but it's harder to do the opposite.