fix matlab build

Author: Lee Carver

atintegrators/BeamLoadingCavityPass.c

@@ -326,7 +326,7 @@ void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
       /*optional attributes*/
       Energy=atGetOptionalDouble(ElemData,"Energy",0.0); check_error();
       z_cuts=atGetOptionalDoubleArray(ElemData,"ZCuts"); check_error();
-      detune_angle=atGetOptionalDouble(ElemData,"detune_angle"); check_error();
+      detune_angle=atGetOptionalDouble(ElemData,"detune_angle",0.0); check_error();
 
       Elem = (struct elem*)atMalloc(sizeof(struct elem));
       Elem->Length=Length;