Add badness calculation

Current using the number of times someone is on-rota two days in a
row, multiplied by the standard deviation in the allocation
frequencies.

rotagen.h

@@ -33,6 +33,7 @@ void generate_rota(struct slot_result *rota_out);
 void generate_allocation(int slot, int job, struct allocation *allocation_out);
 bool satisfies_allocation_constraints(const struct allocation *allocation);
 bool satisfies_slot_constraints(const struct slot_result *slot);
+int rota_badness(const struct slot_result *rota);
 void print_rota(const struct slot_result *rota);
 
 #endif