Add unified file handler for reading / writing

[jacanchaplais]

Combine or refactor reader and writer classes into a factory class, which is passed a mode, so can be used for reading or writing.

[jacanchaplais]

This interface could also introduce usage of attribute assignment syntactic sugar to replace explicit methods for storing data to disk. Explore whether this is possible using set attribute dataclass hooks.

[jacanchaplais]

I am thinking there should be a dramatic simplification of the package. No distinction between reading and writing classes, reading should be allowed during writing, etc.

For the moment, I have added a context manager called open_file() which takes mode as a parameter to provide the relevant interface. It additionally takes an optional parameter process, to jump straight to accessing the process groups in the files.