g.cget return type (for enums?)

On MERLIN:

```
g.cget("Speed_achieved")["value"]
```

Used to return an `int` (`0` or `1`) but now returns a `str` (`"0"` or `"1"`).

As a developer I would like to understand why this changed as I don't think this was an intentional API change?

### Acceptance criteria
- There is a page on the dev wiki explaining what the interactions between enums and genie_python
- There is a page on the user manual detailing that strings should be used over numbers in scripts when dealing with enums
- There should be appropriate tests for the interactions between enums and genie_python