Giving the user the possibility to choose which module to load

[lelaus]

So far, each liquid metal class loads all the available property modules when it is instantiated.
It could be useful giving the user the possibility to select which modules to load, whether all the available ones (as now) or only a few of them.

[lelaus]

Two possible layouts have been identified:

1. by introducing an additional argument into the constructor of the liquid metal, which would allow to select which modules to load at each instantiation;
2. by handling the modules' selection in the same way as the selection of the correlations.

The layout #1 could be useful, but some issues would arise since the loaded modules at the class level would not be synchronized with the ones available at the instance level. Enabling this synchronization would require great modifications to the entire code layout.
The layout #2 is the chosen solution: that is, replicating for the modules what has already been done for the selection of the correlations.

[lelaus]

In addition to what stated in the above comment, the management of the modules' selection should allow also to restore the default modules' selection (in accordance to what should be implemented for the property correlations too: issue #146).