Pdf-essentials

**What's new?**

Aded highlight_text_in_pdf(..), which allows the user to highlight/annotate whole words, substrings or regex patterns in a PDF file and comment these.

**What's new?**

- Integrated additional core functions (PDF/A conversion and validation, image removal from PDF pages)
- Refactoring
- Tests (integrated basic test suite which will be gradually updated over time)

**Initial release**

Nine core functions were implemented, documented and tested:

1. add_page_numbers(..)
2. remove_pdf_metadata(..)
3. rotate_pages(..)
4. anonymize_and_rasterize_strings(..)
5. delete_pdf_pages(..)
6. extract_pdf_pages(..)
7. rearrange_pdf(..)
8. enable_text_copy(..)
9. make_pdf_read_only(..)

Note, make_pdf_read_only(..) doesn't work for all tested pdf types and is subject for improvement.