Optimize reading informations about thermodynamic states from the NetCDF file

[andrrizzi]

There are a couple of further optimizations that can be done here.

1. The temperature of the state is saved separately from the system in the dictionary {system, temperature, pressure} that represents the ThermodynamicState. If we save this dictionary using Reporter._write_dict(..., nested=True), we could add a Reporter.read_temperatures() method and read the temperature without reading the system.
2. We should add to Reporter.read_thermodynamic_states() two arguments (one for sampled and one for unsampled states) specifying the states to read to give more control. This would substitute Reporter.read_end_thermodynamic_states().