0.3.0: New read() semantics

This implements the changes to the read() method we discussed on the Mailinglist and accordingly increments the version number.

It is realized through a change in BaseIO (now returning a Block list instead of a single Block). Only the AlphaOmegaIO and Hdf5IO needed to be changed. I've also adapted the tests and docs to the new behavior.

I have not changed the write() method - we should consider changing it as well, e.g. by allowing a list of Blocks as a parameter and raising an exception if only one block can be saved.