Release 5.6.0

[miekkasarki]

The major version releases modify the contents of the HDF5, meaning that the old files need to be updated first with a5update script before they can be used with the newest version (this could change once the code is run solely through Python).

I'm making this issue here to pin a branch where I'll branch/rebase all other branches that modify the contents of the HDF5 and, hence, belong to 5.6.0 release.