Clean up data model for boxscores

[tturocy]

The boxscore data structure (see src/cwlib/box.h) has one collection of records for batting and fielding, and a separate collection for pitching. While having the separate linked list of pitching stints in order is useful, it would also be useful to have a direct link between CWBoxPlayer entries and corresponding CWBoxPitching entries.

Note that a bit of care may need to be taken in the case of a player who pitches, moves to another position, and then returns to pitcher.