Enabling begin, mid, end preview keywords

[dkazanc]

• adds begin, mid, end keywords to be used with start, stop parameters in preview.
• adds start_offset and stop_offset to be used together with the keywords
• adds tests for keywords and offsets as well as a pipeline where preview is used

Sorry for a couple irrelevant fixes, as they were needed for tests to pass.

Checklist

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have made corresponding changes to the documentation

[yousefmoazzam]

Looks good to me, thanks!

One last thing I might ask of for you (though I'm happy to do it if you're busy): the feature is modifying parse_preview(), which is in transform_loader_params.py, so I think the tests should go in test_transform_loader_params.py.

Could the new tests you've added be moved form test_preview.py to test_transform_loader_params.py please?

There's a few small things regarding types that I'm happy to do myself after that's been done.