Implement get_value_ptr() method for Python

[aufdenkampe]

We noticed that the get_value_ptr() method was not implemented in the Cython example-cxx/pymt_heatcxx/lib/heatcxx.pyx that was built when we followed the draft Babelizer C++ example at https://babelizer.readthedocs.io/en/mdpiper-add-cxx-example/example-cxx.html (see #105 (comment)).

Is there a reason for not implementing this example in Python?

We're presently implementing a BMI Python wrapper for another C++ library, using Cython, and are having challenges with this particular method. We would love to see another working implementation (or understand if there is a good reason to not implement it).