BibFormat: format -> format_element API change
- Format elements (BFE_*) must now define function "format_element()" instead of function "format()", to avoid conflicts with Python "format()" built-in function. Updated guides. Updated provided elements. Backward-compatiblity implemented.
- Renamed 'format' attribute of the BibFormatObject instances to 'output_format'.
- Renamed 'format' parameter of the BibFormatObject init() method to 'output_format'.
- Removed deprecated BibFormatObject attribute 'uid' (has been marked as deprecated already for quite some time).
- Renamed 'format' parameter of the call_old_bibformat() function to 'of'.