Simplify and fix HDF5Attribute (including name fix)

[Prof Garth Wells]

dolfin::HDF5Attribute needs some work:

• Be consistent with types, e.g. do not confuse int and std::size_t
• Use boost::variant to reduce substantially the number of interfaces
• Considering re-naming to better reflect what the class does.

We can even consider the need for this class. From Python, h5py is good tool.