AP_HAL_SITL: compiler warning: float to double promotion

- replace atof() with strtof() for the correct return time

libraries/AP_HAL_SITL/SITL_State.cpp

@@ -32,7 +32,7 @@ void SITL_State::_set_param_default(const char *parm)
         printf("Please specify parameter as NAME=VALUE");
         exit(1);
     }
-    float value = atof(p+1);
+    float value = strtof(p+1, NULL);
     *p = 0;
     enum ap_var_type var_type;
     AP_Param *vp = AP_Param::find(pdup, &var_type);

libraries/AP_HAL_SITL/SITL_cmdline.cpp

@@ -176,7 +176,7 @@ void SITL_State::_parse_command_line(int argc, char * const argv[])
             model_str = gopt.optarg;
             break;
         case 's':
-            speedup = atof(gopt.optarg);
+            speedup = strtof(gopt.optarg, NULL);
             break;
         case 'F':
             _fdm_address = gopt.optarg;