Implement the feature of MS+MS mergers from Schneider et al.

Implement the merging prescription of Schneider et al. would be done by @dimsour94 and @ezapartas and is probably aimed for a version of POSYDON after v2.0. We need the avg. H mass fraction of the MS stars merging and the matched stars for this which will be calculate from total_mass_h1. "There they take into account the rejuvanation process. The implementation appears straightforward, as it only requires the average hydrogen mass fraction of both stars before merging and their initial hydrogen mass fraction. Using this information, we can estimate the average hydrogen mass fraction of the merger product and then align it with our single-star model that matches both the mass and average hydrogen mass fraction of the merged product"