Add file create data appending

Close #990.

Requires https://github.com/hdmf-dev/hdmf/pull/280.

@rly @t-b what's the status of this?

We need to find a way to tell pynwb that certain datasets in HDF5 need to be written as chunked by default. Only then they are appendable. I don't know how to do that.

@t-b ah, ok. Sounds like a job for H5DataIO