dummy_derivatives: Reset eqcolor every loop

We're using augmenting path construction here to try to find a maximal
matching of the variables at the given differentiation level. However
the construct_augmenting_path! function assumes as a pre-condition that
eq-color is reset to zero in order for it to actually produce an
augmenting path. Failing to do this can cause it to falsely declare
a system singular.

Author: Keno

src/structural_transformation/partial_state_selection.jl

@@ -227,6 +227,7 @@ function dummy_derivative_graph!(structure::SystemStructure, var_eq_matching, ja
             else
                 rank = 0
                 for var in vars
+                    eqcolor .= false
                     # We need `invgraph` here because we are matching from
                     # variables to equations.
                     pathfound = construct_augmenting_path!(rank_matching, invgraph, var,