Fix memory error when deallocating Python reaction rate wrappers

[speth]

Using the value of member variables that are Python objects in __dealloc__ is not reliable, because they may have already been
deleted by the time __dealloc__ is called [Cython docs]. Instead, we need a C data member to check for whether or not the rate object owns the underlying C++ object and should delete it.

The only way I've been able to trigger this bug is interactively in IPython, when trying to access members of the rate object with tab autocompletion. I think the reason for this is that this scenario ends up invoking the garbage collector, rather than having objects just get deleted when their reference counts go to zero, and that ends up behaving differently. I'm honestly surprised I didn't run into this before -- this is the way this has worked as long as this information has been accessible in Cython.

Changes proposed in this pull request

• Add C++ boolean own_rate to Cython Arrhenius and BlowersMasel classes and use this when checking whether to delete the pointer to the corresponding C++ objects.

Checklist

• There is a clear use-case for this code change
• The commit message has a short title & references relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[ischoegl]

Interesting - fix looks good to me.