Made LiquidMetalInterface instance pickable

[lelaus]

The reason why a LiquidMetalInterface instance was not pickable was because a lambda function was used in the __add_property() method.

The solution implemented is to replace such lambda function passed to setattr() in the __add_property() method with the private __property_info() method of the LiquidMetalInterface class. This way pickle can reference the name of such private method. This also implies the need to use functools.partial to define the method to add to the LiquidMetalInterface instance in __add_property(), so that you can only define the method to call and not call it.