Add a mission_xp value to Item_type - how much XP to reward for missions that use this item.
Add Data_pool::add_element(); Data_pool::load_element() lets this function do most of the work now.
Comment
Allow reading of books if they won't teach us and we can't gain morale BUT it's related to a mission.  Assign new personal missions when we clean up missions.  Optionally display a message about our new personal missions when they're assigned.
Greatly improve Mission::get_description().  New method Mission_template::base_on_item_type() automatically creates a Mission_template centered on the given type.
No loading mission templates from a data file (that was a dumb idea).  Instead, automatically generate a Data_pool of Mission_template based on existing Item_types, book genres, and more...?  Yes, more, later.
In an attempt to ensure all real (e.g. not an empty weapon slot) items have a type and a UID, make Item::type private.  This is currently preventing successful compilation.