BibFormat: parameter adjustments in bfe_abstract

* Adds the function extend_max_chars, which increases the max_char
  parameter in case the abstract contains mathematic formulas, thus
  preventing a abstract substring that ends in the middle of a
  formula, which would lead to a false rendering.

* Adds the function split_regarding_substrings which splits a string
  except for those substrings, which are found with given regex
  patterns. Thus preventing the limit parameter to split the abstract
  in a mathematical formula (eventhough this might never happen).

* Changes the test test_get_tags_used_by_element to use another file
  (bfe_additional_report_numbers.py), since the refactoring of
  bfe_abstract.py makes it fail. The searched tags are now stored
  as variables, wherefore they are not found anymore (see
  bibformatadminlib.get_tags_used_by_element).

Signed-off-by: Martin Vesper <martin.vesper@cern.ch>