Create documentation about when+what to purge

We spent a while trying to debug after adding a property today bur not purging. We didn't immediately realize we needed to purge because we were able to access the property with no errors, but it was empty. Ideally we would recompile the python when the properties of an algorithm are changed. The next best option is to throw an error when trying to access a property that has not been filled. If those are both too difficult, we think that documentation about when to purge would be useful. In the documentation ideally it would be explained what needs to be purged in different scenarios to avoid a full purge when possible.