Reduce file size

[avdata99]

The file src/okfn_iati/multi_csv_converter.py is huge

We have a class to process activity files
Also, we have the two ways included XML to CSV and CSV to XML

This is a lot

We have a lot of extractors

And we have a lot of builders

We need to split this into multiple files.
This is a plan, it could be different.

We'll love to see:

• an activities folder in src/okfn_iati
• an process_xml subfolder in the activities folder
• an process_csv subfolder in the activities folder
• All functions related to xml to csv lives in multiple files process_xml subfolder
• All functions related to csv to xml lives in multiple files process_csv subfolder
• All common code remains in the base class

We don't expect any code improvement or change. Just break this file down into multiple part so it's easy to develop.