as.data.frame method

**NOTE: This is out-of-scope but I need somewhere to put this reminder**

There are times when it's necessary to `as.data.frame(HDF5Matrix)`. This goes via `as.array(HDF5Matrix)`, necessitating 2 conversions (_HDF5Matrix_ -> _array_ -> _data.frame_) and associated memory allocations. A more efficient implementation would fill the _data.frame_ directly form the HDF5 file.