fix(engine_multiscale): resolved bug which prevented proper equilibrium abundances from being found

this was done by adjusting the scaling of the QSE operator() residuals from r = dy/dt to r=(dy/dt)/y

src/include/gridfire/utils/general_composition.h

@@ -0,0 +1,21 @@
+#pragma once
+#include "fourdst/composition/composition.h"
+#include "fourdst/composition/atomicSpecies.h"
+
+namespace gridfire::utils {
+    inline double massFractionFromMolarAbundance (
+        const fourdst::composition::Composition& composition,
+        const fourdst::atomic::Species& species,
+        const double Yi
+    ) {
+        double sum = 0;
+        for (const auto& [symbol, entry] : composition) {
+            if (entry.isotope() == species) {
+                sum += species.mass() * Yi;
+            } else {
+                sum += entry.isotope().mass() * composition.getMolarAbundance(symbol);
+            }
+        }
+        return (species.mass() * Yi) / sum;
+    };
+}