Added call to plumed_finalize in cython dealloc routine.

This should ensure that plumed_finalize is called when the python/cython
class is destroyed

Addresses #295

python/cplumed.pxd

@@ -27,3 +27,4 @@ cdef extern from "Plumed.h" :
          pass
      plumed plumed_create()
      void plumed_cmd(plumed p, const char*key, const void*val) except + 
+     void plumed_finalize(plumed p) except +

python/plumed.pyx

@@ -35,8 +35,7 @@ cdef class Plumed:
          cdef int pres = 8
          cplumed.plumed_cmd(self.c_plumed, "setRealPrecision", <void*>&pres )  
      def __dealloc__(self): 
-         pass
-         #del self.c_plumed
+         cplumed.plumed_finalize(self.c_plumed)
 
      def cmd_ndarray_real(self, ckey, val):
          cdef double [:] abuffer = val.ravel()