tearing: Heuristically attempt to assign equations with single solvable first

This changes to tearing to give priority to any equation <-> variable pairs
where a particular equation only has one matchable variable. These are easy,
becuse, if the equation is part of the final matching at all, we know which
variable they need to be matched to and by matching them first, we can avoid
accidentally assigning these variables to another equation, which would
necessarily increase the number of torn states.

Author: Keno

src/structural_transformation/bipartite_tearing/modia_tearing.jl

@@ -32,17 +32,22 @@ function tearEquations!(ict::IncrementalCycleTracker, Gsolvable, es::Vector{Int}
             end
         end
     end
-    for eq in es  # iterate only over equations that are not in eSolvedFixed
-        vs = Gsolvable[eq]
-        if check_der
-            # if there're differentiated variables, then only consider them
-            try_assign_eq!(ict, vs, v_active, eq, isder)
-            if has_der[]
-                has_der[] = false
-                continue
+    # Heuristic: As a first pass, try to assign any equations that only have one
+    # solvable variable.
+    for only_single_solvable in (true, false)
+        for eq in es  # iterate only over equations that are not in eSolvedFixed
+            vs = Gsolvable[eq]
+            ((length(vs) == 1) ⊻ only_single_solvable) && continue
+            if check_der
+                # if there're differentiated variables, then only consider them
+                try_assign_eq!(ict, vs, v_active, eq, isder)
+                if has_der[]
+                    has_der[] = false
+                    continue
+                end
             end
+            try_assign_eq!(ict, vs, v_active, eq)
         end
-        try_assign_eq!(ict, vs, v_active, eq)
     end
 
     return ict