Simplify Python interface to HDF5 file by using H5Py

[Prof Garth Wells]

Use http://www.h5py.org/. No point re-inventing the wheel.

[Tormod Landet]

NOTE: importing a h5py package that has been pip installed will often segfault due to incompatible HDF5 libs between h5py and dolfin. We should install h5py in the docker container with

pip3 install --no-binary=h5py h5py

if we chose to make h5py the go-to method for more advanced HDF5 wrangling.

I have tried to make a foolproof way of detecting whether h5py is incompatible so that a warning can be printed about this at an appropriate time instead of the interpreter dying when importing h5py, but I have not found a good way to do this. It is not possible to handle compiled module SIGSEGV in Python