change default lambda_ce to 0.5 because setting it to -1 is not

valid over the whole parameter space, especially not at Z<0.02

src/binary_c_parameters.h

@@ -1093,9 +1093,14 @@
 
 /*
  * Lambda is the envelope binding energy parameter:
- * Ebind = -G M Menv / (R lambda) 
+ * Ebind = -G M Menv / (R lambda)
+ *
+ * Note: much as we'd like to set this to one of the 
+ *       three LAMBDA_* macros (for the Dewi/Tauris, 
+ *       Wang or Polytrope) these are not reliable over
+ *       the whole parameter space. Instead, use 0.5.
  */
-#define DEFAULT_LAMBDA_CE -1.0
+#define DEFAULT_LAMBDA_CE 0.5
 
 /* use Hurley et al. (2002) energy balance prescription by default */
 #define DEFAULT_COMENV_PRESCRIPTION COMENV_BSE