Fix remaining memory leaks

[jesper-friis]

Running make memcheck currently shows no memory errors and 3 memory leaks:

60/120 MemCheck: #60: test_python_bindings-py ..........   Defects: 2
75/120 MemCheck: #75: test_mapping-py ..................   Defects: 1
93/120 MemCheck: #93: test_mongodb_python ..............   Defects: 1

Leak 60 is a duplicate of leak 73 and 93.

Leak 75
Require implementation of function dlite_mapping_plugin_free().
See the comment on line 104 in src/dlite-mapping.c

Leak 93
To be investigated more.
Seems that Python is not calling the destructor for StorageIterator in bindings/python/dlite-storage.i.

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When these issues are fixed, and cmake --build . --target memcheck runs without failure, consider to add a GitHub CI test running cmake --build . --target memcheck for selected Python versions and platforms. However, please note that the memory checking is very slow, so it should not be run too often. Maybe periodically or only when merging to master?

Is it possible to save the log files, Testing/Temporary/MemoryChecker.<#>.log, somewhere where they can be accessed for later inspection? Alternatively make the output verbose by removing the -q option to valgrind.

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For debugging, use the following command to rerun the individual tests with valgrind:

DLITE_ATEXIT_FREE= valgrind --tool=memcheck --leak-check=yes --show-leak-kinds=definite,indirect --errors-for-leak-kinds=definite --num-callers=200 --keep-debuginfo=yes --suppressions=cmake/valgrind-python.supp --suppressions=cmake/valgrind-dlite.supp --gen-suppressions=all python bindings/python/tests/test_mapping.py