What is the average capacity for the vehicle having a navigator of Macneall?
CREATE TABLE "modern_competition" (
  "driver" text,
  "navigator" text,
  "vehicle" text,
  "class" text,
  "capacity" real,
  "total_time" text,
  "margin" text
);
SELECT AVG("capacity") FROM "modern_competition" WHERE "navigator"='macneall';