Fix surrounding context extraction in ParsrConverter

[bogdankostic]

This PR fixes the extraction of surrounding contexts of tables in the ParsrConverter. Previously, the surrounding context was always empty if remove_page_headers or remove_page_footers was set to False. Also, preceding_context_len and following_context_len was not taken into account.

[Timoeller]

I tested the changes and our Parsr implementation now returns the context.

It does not return sentences but whole lines, so if a single line consists of many sentences a lot of text is returned.
Please open another issue for this to keep for later. I don't think we should work on this now.

But I would like to see in both Parsr and Azure some tests for the additional metadata extraction, especially since we are using external tools that might change return types without our notice. Can you please add some tests for the metadata extraction?

[bogdankostic]

I added the tests and created issue #2217. Furthermore, I unified ParsrConverter's and AzureConverter's behavior. (ParsrConverter was previously returning whole paragraphs instead of lines as surrounding context.)

[Timoeller]

LG