ref_sphere computes the radius of a reference sphere just outside a Roche distorted star
along the line of centres and centred upon its centre of mass. This sphere, which is guaranteed
to enclose the equipotential in question can then be used to define regions for searching for
equipotential crossing when computing eclipses. The Roche-distorted star is defined by the mass
ratio and the (linear) filling factor defined as the distance from the centre of mass of the
star to its surface in the direction of the L1 point divided by the distance to the L1 point.
A filling factor = 1 is Roche filling. Note that the size of the Roche lobe is calculated as
appropriate given any asynchronism.