Fix memory leak caused by reader/writer objects not being garbage collected during pipeline execution

[yousefmoazzam]

Fixes #264

[yousefmoazzam]

The reason why reader/writer objects weren't being garbage collected during pipeline execution

Following on from seeing this helpful stackoverflow post, further inspection of the documentation for weakref.finalize() brought the following warning to attention:

This explains the issue! The functions being passed to weakref.finalize() are indeed "bound methods" on the objects being passed in:

• if reader is an instance of DataSetStoreReader, then the bound method reader.finalize is passed in to weakref.finalize(), which contains a reference to the reader object, incurring the consequences of the above warning
• if writer is an instance of DataSetStoreWriter, then the bound method writer.finalize is passed in to weakref.finalize(), which contains a reference to the writer object, incurring the consequences of the above warning

Therefore, due to passing references to bound methods of the objects to be "finalized", the objects in question (reader and writer objects) were destined to never be garbage collected until the program terminated.

Confirmation of that particular memory leak going away with this fix

Here is the original memory leak from the original issue for easy comparison:

After swapping out the regular references to the bound finalize methods on the reader/writer for weak references using weakref.WeakMethod, the RAM usage appears to be as expected for the 20GB data when using the pipeline described in #264: