Checkpointing for ADC calculations

[mfherbst]

Since large ADC calculations may take a considerable time it would be nice to checkpoint or archive it for later use. This roughly requires the following

• A way to serialise a libtensor Tensor object to hdf5
  - Serialise the symmetry information
  - Serialise the data (using ideally only the canonical blocks)
• A way to load a libtensor Tensor object from hdf5
  - Basically undo the above
• A way to in serialise the symmetry setup of a particular calculation
• A way to load the symmetry setup of a particular calculation and reuse it.