Introduce the ability to restore the default correlations

[lelaus]

So far, methods are already implemented for managing the property correlations at the liquid metal class level:

• properties_for_initialization() = it returns all the properties that can be used for instantiating a liquid metal object;
• available_correlations() = it returns the dictionary providing the list of the available correlations for each available property;
• set_correlation_to_use() = it sets the correlation to use for the specific property passed as argument; this method acts at the class level, thus applying to any object instantiated from that point on;
• correlations_to_use() = it returns the dictionary defining the correlation used for the properties for which a specific correlation has been chosen a priori.

A feature is missing among all the above methods, that is, the possibility to restore the default correlations. It could be useful to implement such a method.