added HDF5 decoder

[wedeling]

Added an HDF5 decoder, which is more flexible than the CSV decoder. Allows to load multiple QoIs of different size from the same HDF5 file. Syntax is the same as for the CSV decoder:

decoder = uq.decoders.HDF5(
    target_filename='output.hdf5',
    output_columns=['array_of_size10', 'array_of_size1000'])

New files:

• easyvvuq/decoders/hdf5.py: the HDF5 decoder
• test/test_decoder_hdf5.py: a test for the HDF5 decoder

This uses the h5py library, which I added to requirements.txt

[DavidPCoster]

Looks good! I think this adds significant functionality!

[djgroen]

This adds an important functionality, with minimal risk to the rest of the code base. I don't see any obvious clashes with existing elements, and the code looks sound to me.