Add possibility of starting the run from a user provided configuration

[ec147]

• Addition of new solve parameter initial_configuration to start the run from a user-provided configuration.
• The current configuration is now stored in S.configuration.

An usage example to restart a run from the last configuration would be:

S.solve(...)
config = S.configuration
S.solve(...,initial_configuration=config)

The Python class Configuration has also been added, and the functions h5::write and h5::read have been modified/added such that a configuration can be stored and read from a hdf5 file.