Support for h5py special types

1. https://h5web.panosc.eu/?file=dtype.h5
2. Select `specified` to show datasets created with specific dtypes.

The datasets created with one of h5py's [special types](https://docs.h5py.org/en/stable/special.html) cannot be fetched (500 server error):

![image](https://user-images.githubusercontent.com/2936402/113699089-6c0e2980-96d5-11eb-9561-e62462db888e.png)
