Fixes #212

Proposed Changes

• Drop melting points from PhaseTransition for carbon allotropes (graphite and diamond) since they are not at starndard pressueres.
• Add is_sublimation_point to PhaseTransition where boiling point is a sublimation point
• Add is_transition to PhaseTransition where melting point is a phase transition to the crystalline form.
• Element.melting_point and Element.boiling_point consistently return either a float or None

After the changes

>>> from mendeleev import C
>>> print(C.melting_point)
None
>>> print(C.boiling_point)
4098.15
>>> print(C.phase_transitions)
[PhaseTransition(atomic_number=6, allotrope=graphite, melting_point=None, boiling_point=4098.15, triple_point_temperature=4762.15, triple_point_pressure=10300.0, critical_temperature=None, pritical_pressure=None, is_sublimation_point=True, is_transition=None)]