MgMn phase diagram

Degeneracy probe for the MP phase-diagram route: compound input at a composition with (expected) no stable MP compound. If the Mg-Mn hull has no compound competition, the route must return eabovehull == formationenergyper_atom exactly (hull reduces to the elemental tie-line), demonstrating the definitional equality case on a compound input. Parsed Pm-3m, 2 atoms, a=3.00 Å, min dist 2.598 Å.