Check for L2 overflow

[ilyamandel]

Currently, we check whether stars are touching (e.g., left touching by an MT episode), and use this as a proxy for merger. This check resolves to a check for whether the sum of the stellar radii exceeds the semi-major axis:

BaseBinaryStar.h: bool HasStarsTouching() const { return (utils::Compare(m_SemiMajorAxis, 0.0) > 0) && (m_SemiMajorAxis <= RSOL_TO_AU * (m_Star1->Radius() + m_Star2->Radius())); }

A more realistic check would consider whether stars engage in L2 overflow. The two are equivalent for equal-mass stars in a circular binary, so the current check is not a terrible proxy, but we should improve this.

P.S. Should also address the initial check in the constructor of BaseBinaryStar (see #309 ).