* implement support for XCTYPE
* implement support for multiple include directories
* `.parse()` is now fully idempotent (but not re-entry safe)
* the parser object has now a well-defined state after parsing
  (requirement for using it as a completion engine)
* internal representation change to a tree with dataclasses,
  and nested dicts (canonical and simplified) are now "views" of that
* adding method to obtain parsed (and unit converted) `COORD` section
* preprocessor is now a proper iterator (returning pre-processed lines)
  with access to its internal state (requirement for improved error messages)
* more tests