Handle DARRAYs of real variables

This involves working out the code to get the base type of a select
expression of a darray. Also added the runtime support for darrays
with real value elements.

eval_tree.cc

@@ -53,6 +53,9 @@ static bool get_real_arg_(const NetExpr*expr, verireal&val)
 		break;
 	  }
 
+	  case IVL_VT_DARRAY:
+	    return false;
+
 	  default:
 	    assert(0);
       }