Support H5FD_CORE file driver for in-memory files

[Vindaar]

H5FD_CORE is a file driver that is part of the HDF5 library, which creates files purely in memory.

This can be useful to e.g. create a file and send it via socket without having to write it to disk.

See:

• https://support.hdfgroup.org/HDF5/doc1.8/RM/RM_H5P.html

Two things:

• figure out how to open file with driver (should just be an argument to the file access property list, I imagine), i.e. replace H5D_DEFAULT by some custom thing
• understand how to access the full data in memory somehow (start and size?).