[Cython] Expose global 'appdelete' method

Occasionally useful for debugging memory leaks

interfaces/cython/cantera/_cantera.pxd

@@ -18,6 +18,7 @@ cdef extern from "cantera/base/ctml.h" namespace "ctml":
 cdef extern from "cantera/base/global.h" namespace "Cantera":
     cdef void CxxAddDirectory "Cantera::addDirectory" (string)
     cdef size_t CxxNpos "Cantera::npos"
+    cdef void CxxAppdelete "Cantera::appdelete" ()
 
 cdef extern from "cantera/thermo/mix_defs.h":
     cdef int thermo_type_ideal_gas "Cantera::cIdealGas"

interfaces/cython/cantera/utils.pyx

@@ -24,3 +24,7 @@ def _have_sundials():
     return bool(get_sundials_version())
 
 __version__ = pystr(get_cantera_version())
+
+def appdelete():
+    """ Delete all global Cantera C++ objects """
+    CxxAppdelete()