Started the initiative for using more DCGs in SCG, STCN, and DBNL.

* [CODES_EXT,NEW] New module, contains split_codes/3 and strip_codes/3.
* [DBNL_EXTRACT,EXT] Many extensions for parsing titles from the index. This has to move to the DCG modules as much as possible.
* [DBNL_INDEX,EXT] Better parsing of titles from the index, asserting more information.
* [DCG_ASCII,NEW] Partial copy of module ASCII with simplified DCGs.
* [DCG_CARDINAL,NEW] DCGs for cardinal numbers. Currently contains binary, hexadecimal, decimal, and octal digits.
* [DCG_CENTURY,NEW] DCGs for parsing centuries.
* [DCG_GENERIC,NEW] Module intended for generic DCG clauses.
* [DCG_ORDINAL,NEW] DCGs for ordinal numbers.
* [DCG_YEAR,NEW] DCGs for years and year intervals, taking uncertainty and various notations into account.
* [META_EXT,NEW] Added xor/2.
* [BOOLEAN_LOGIC,NEW] New module implementing Boolean logic (at some point...). Currently only contains conjunction.
* [DBNL,DCG,SC] Quite some cleaning of the code for parsing the title entitries in the index.
* [DCG_PRINT,NEW] New module for parsing publication print numbers and information about the print. Very specific to DBNL right now, make more generic...