Compressed i/o
This adds gzip compression to the datasets within the HDF5 outputs. That can save a lot of storage space for arrays that are constant (mass,...) or similar/regular (IDs, h, ...).
We can play with the chunk sizes and compression levels later on, when we have "real" runs and a parameter file.
It "solves" issue #106 (closed).
Feel free to remove the source branch.