Inventory system changes caused by new objects

Inventory system now works with new objects.  All objects: episode,
exchange, cell, screen, token are working.
Inventory object

Inventory object added; basic functionality in place.  Object holds
both the free and stored inventory; it’s starting state is read in from
the db files.  Code to move tokens in/out of stored inventory not yet
fixed.

Other inventory changes made:  teacher stored inventory is now per
guide, not per screen.  guideID property added to screen object.
guideID field added to screen db recs.  All stored inventory is based
on episodeID and guideID.  Learner is permanently assigned as ID: g0.
(g zero)