add ability to retrieve the number of local rows for computes returning local data

this is done supporting the combination of type == 2 with style == 0,
i.e. a local scalar, which is not available in C++ (but there one can
access the compute style data member directly. for the python interface,
the pointer is automatically dereferenced and returned as a c_int.

python/lammps.py

@@ -384,9 +384,14 @@ class lammps(object):
       ptr = self.lib.lammps_extract_compute(self.lmp,id,style,type)
       return ptr
     if type == 2:
-      self.lib.lammps_extract_compute.restype = POINTER(POINTER(c_double))
-      ptr = self.lib.lammps_extract_compute(self.lmp,id,style,type)
-      return ptr
+      if style == 0:
+        self.lib.lammps_extract_compute.restype = POINTER(c_int)
+        ptr = self.lib.lammps_extract_compute(self.lmp,id,style,type)
+        return ptr[0]
+      else:
+        self.lib.lammps_extract_compute.restype = POINTER(POINTER(c_double))
+        ptr = self.lib.lammps_extract_compute(self.lmp,id,style,type)
+        return ptr
     return None
 
   # extract fix info