add nonstellar timestep function which can be called from outside stellar_timestep

src/timestep/nonstellar_timestep.c

+#include "../binary_c.h"
+No_empty_translation_unit_warning;
+
+#include "timestep.h"
+
+void nonstellar_timestep(struct stardata_t * const stardata,
+                         double * const dtm)
+{
+    Foreach_star(star)
+    {
+        timestep_fixed_timesteps(stardata,
+                                 star,
+                                 dtm);
+    }
+
+    /*
+     * Hard wired minimum : do not apply
+     * if the timestep is limited by a fixed timestep
+     */
+    Foreach_star(star)
+    {
+        if(stardata->model.fixed_timestep_triggered == FALSE &&
+           star->dtlimiter != DT_LIMIT_FIXED_TIMESTEP)
+        {
+            *dtm = Max(*dtm,
+                       MINIMUM_STELLAR_TIMESTEP);
+        }
+    }
+
+}