Add impurities class

[lelaus]

The aim is to implement a new class, at the same level of liquid metal classes, which provides data and correlations about irradiation-induced impurities. This class provides information that are independent of the liquid metals, thus requiring to be instantiable alone.

This issue is focused on a specific task identified in issue #183.

[lelaus]

From e969e08, the following tasks must be accomplished yet:

• to improve the structure of the new classes;
• to update the tests accordingly;
• to update the doc accordingly;
• to think about the need of enabling the addition of new impurities and of new custom properties.