Functions to simplify saving persistent settings

[jtc42]

Things

We should have some functions to simplify loading and updating settings files. See https://gitlab.com/openflexure/openflexure-microscope-server/-/blob/master/openflexure_microscope/config.py

Extensions

We should have a LabThings-level settings file, with keys corresponding to extension names. Extensions can then really easily load and save persistent settings to this file.

[jtc42]

Out of scope. Other libraries exist to facilitate this.