Implementation of pure lead contamination's tests

[prisma34]

Implement tests for initialization and boundary values verification for contaminating compounds in pure lead.
It involves the implementation of the LeadContaminationTester class, with:

• the test_init_fromX method in test_lead_fromX_spanT.
• the test_min method in test_lbh15_bounds.py.
• the test_max method in test_lbh15_bounds.py.
• the test_T_at_min method in test_lbh15_bounds.py.
• the test_T_at_max method in test_lbh15_bounds.py.

This issue is handled as one of the tasks of issue #188.

[lelaus]

A few properties of contaminants provide a constant value. This disable the possibility to initialize a liquid metal object by providing the value of such properties to the constructor.
In the method _compute_T() this aspect must be managed appropriately. For the time being, a check requiring several computations of the property correlations is implemented to identify whether the correlation provide a constant value. Although correct, this procedure increases the effort of the code, thus reducing its computational efficiency.
To reduce this increase of effort, I'd suggest to add the method is_constant() to the abstract class PropertyInterface, which by default returns False. In each property class whose correlation provide a constant value, such method must be overridden by making it return True. In this way, in _compute_T() the only check to perform is about the value returned by the method is_constant().

[prisma34]

Tasks from the second to the last are handled in issue #188.

[lelaus]

The first task of this issue has been managed. Since constant properties are added, the method is_constant() has been implemented in the class PropertyInterface to quickly identify such properties.

No need to change the doc.

Tests for lead contaminant properties have been added.