fix: #114 improve robustness to unknown boundry conditions

Author: ShijieYan

src/mcx_utils.c

@@ -1188,11 +1188,11 @@ void mcx_preprocess(Config *cfg){
     cfg->srcdir.z*=tmp;
 
     for(int i=0;i<6;i++)
-        if(cfg->bc[i]>='A' && mcx_lookupindex(cfg->bc+i,boundarycond))
+        if(cfg->bc[i] && mcx_lookupindex(cfg->bc+i,boundarycond))
 	   MCX_ERROR(-4,"unknown boundary condition specifier");
 
     for(int i=6;i<12;i++){
-        if(cfg->bc[i]>='0' && mcx_lookupindex(cfg->bc+i,boundarydetflag))
+        if(cfg->bc[i] && mcx_lookupindex(cfg->bc+i,boundarydetflag))
 	   MCX_ERROR(-4,"unknown boundary detection flags");
 	if(cfg->bc[i])
 	   isbcdet=1;