Simplify get records

[alifeee]

closes #1367

For discussion, see #1367

I made the proposed changes, apart from

• did not rename expected_headers
  I think it will be easier for all if this stays. I only see it potentially being annoying compared to ignore_duplicate_headers if the headers are unknown, in which case we can tell the user to use worksheet.get("1:1") to fetch the headers, to use in expected_headers
• did not remove empty2zero
  it was used in numericization, I moved it under the numericization kwarg to reflect the relation

And:

I did not implement utils.to_records, as @lavigne958 expressed desire to do this