use C++ style include files in USER-PTM package consistently

src/USER-PTM/ptm_canonical_coloured.cpp

@@ -1,4 +1,4 @@
-#include <string.h>
+#include <cstring>
 #include <climits>
 #include <algorithm>
 #include "ptm_graph_tools.h"

src/USER-PTM/ptm_convex_hull_incremental.cpp

@@ -1,6 +1,6 @@
 #include <cmath>
 #include <cfloat>
-#include <string.h>
+#include <cstring>
 #include <cassert>
 #include <algorithm>
 #include "ptm_convex_hull_incremental.h"

src/USER-PTM/ptm_graph_tools.cpp

@@ -1,4 +1,4 @@
-#include <string.h>
+#include <cstring>
 #include <algorithm>
 #include "ptm_graph_tools.h"
 #include "ptm_constants.h"

src/USER-PTM/ptm_index.cpp

@@ -1,6 +1,6 @@
 #include <cstdio>
 #include <cstdlib>
-#include <string.h>
+#include <cstring>
 #include <cmath>
 #include <cfloat>
 #include <cassert>

src/USER-PTM/ptm_initialize_data.cpp

@@ -1,6 +1,6 @@
 #include <cstdio>
 #include <cstdlib>
-#include <string.h>
+#include <cstring>
 #include <cmath>
 #include <cfloat>
 #include <cassert>

src/USER-PTM/ptm_polar.cpp

@@ -87,7 +87,7 @@
 
 #include <cmath>
 #include <algorithm>
-#include <string.h>
+#include <cstring>
 #include "ptm_quat.h"
 #include "ptm_polar.h"
 

src/USER-PTM/ptm_quat.cpp

@@ -1,4 +1,4 @@
-#include <string.h>
+#include <cstring>
 #include <cmath>
 #include <cfloat>
 

src/USER-PTM/ptm_structure_matcher.cpp

@@ -1,6 +1,6 @@
 #include <cstdio>
 #include <cstdlib>
-#include <string.h>
+#include <cstring>
 #include <cmath>
 #include <cfloat>
 #include <cassert>