Add a native Python incremental file reader utility

[lohedges]

This PR adds a simple native Python incremental file reader utility, replacing existing functionality from an external dependency. Since we were only using a minor subset of the functionality, it made sense to reimplement ourselves. The update also makes sure that only full lines are read as new records and moves the state to being in memory on the Process object, rather than living in an offset file on disk, so there are no leaks with different process objects sharing a working directory.

• I confirm that I have merged the latest version of devel into this branch before issuing this pull request (e.g. by running git pull origin devel): [y]
• I confirm that I have added a test for any new functionality in this pull request: [y]
• I confirm that I have added documentation (e.g. a new tutorial page or detailed guide) for any new functionality in this pull request: [y]
• I confirm that I have permission to release this code under the GPL3 license: [y]