Fixing naming of IO module

[WillJRoper]

The IO module name clashed with the python io module. This is now grid_io to differentiate it.

I have also introduced sys.path amending to make it importable in the scripts it was demoed in but these scripts are not updated with errors looking for a tag that does not exist in utils. However, they at least serve as a demonstration.

This needs to be fixed with an installation to make the reusable chunks importable anywhere in the package.

I also fixed an issue where the GridFile looked for a function which has since been attached to a Sed in Synthesizer.

In general, we can probably gain a lot by packaging up internal parts of synthesizer-grids in synthesizer objects.

Issue Type

• Bug

Checklist

• [x] I have read the [CONTRIBUTING.md]()
• [x] I have added docstrings to all methods
• [x] I have added sufficient comments to all lines
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no pep8 errors
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes

[sophie-newman]

you need to import sys in the files you've updated the path in