Refactor process_and_combine.py

[dfulu]

The process_and_combine.py file has two versions of the function

• process_and_combine_datasets() which is only used in PVNetUKRegional
• process_and_combine_site_sample_dict() which is only used in SitesDataset

I think process_and_combine_datasets() and process_and_combine_site_sample_dict() should either be refactored to share more code (there is a lot of copy-pasting going on) or they should be moved into pvnet_uk_regional.py and site.py respectively to match their specialised usage.