append functionality for write_hdf5

[lbluque]

This addresses #29 for write_hdf5. Opening this PR for comments and suggestions for including append functionality.

[lbluque]

For .npy files there is no clean way of doing it. But I doubt users will want to write a very large array (that requires chunking) into a .npy file (which can only be loaded completely, hence defeating the purpose of saving memory). So I suggest to not even go down that road.
Butit would be nice to include append functionality in write_dxf

[decarlof]

@lbluque thank you for your contribution. I will look at the write_dxf. The dxfile at dxfile subclasses the h5py module so should already handle the append.