[Python] Fix premature deallocation when slicing Solution objects

Testing the value of self.parent in _SolutionBase.__dealloc__ was ineffective
because this attribute can already be cleared when __dealloc__ is called,
causing the borrowed C++ objects to be deleted prematurely. A C data member
needs to be used to determine whether this object is responsible for deleting
the underlying C++ objects.

interfaces/cython/cantera/_cantera.pxd

@@ -747,6 +747,7 @@ cdef class _SolutionBase:
     cdef int thermo_basis
     cdef np.ndarray _selected_species
     cdef object parent
+    cdef cbool is_slice
 
 cdef class ThermoPhase(_SolutionBase):
     cdef double _mass_factor(self)

interfaces/cython/cantera/base.pyx

@@ -9,7 +9,8 @@ cdef class _SolutionBase:
 
             # keep a reference to the parent to prevent the underlying
             # C++ objects from being deleted
-            self.parent = other
+            self.parent = origin
+            self.is_slice = True
 
             self.thermo = other.thermo
             self.kinetics = other.kinetics
@@ -19,6 +20,8 @@ cdef class _SolutionBase:
             self._selected_species = other._selected_species.copy()
             return
 
+        self.is_slice = False
+
         if infile or source:
             self._init_cti_xml(infile, phaseid, phases, source)
         elif thermo and species:
@@ -128,7 +131,7 @@ cdef class _SolutionBase:
 
     def __dealloc__(self):
         # only delete the C++ objects if this is the parent object
-        if self.parent is None:
+        if not self.is_slice:
             del self.thermo
             del self.kinetics
             del self.transport