Allow zero probs option in pnr lot choice

[dhensle]

Fix for #1122

• Choosers with an available lot continue to receive a valid lot choice.
• Choosers with no available lot receive pnr_zone_id = -1.
• Tests cover mixed available and unavailable choosers, an empty lot list, and both probability simulation and explicit error terms.
• Documentation explains how to make a lot unavailable in the specification and how tour mode choice should handle pnr_zone_id = -1.