Haverhill — On the first warm, sunny weekend of spring, about 30 people sat inside Haverhill Cooperative Middle School brainstorming about the best use for Pearson Hall, the 197-year-old Federal-style building that once housed Haverhill Academy and the Grafton County Court. Among them was 85-year-old Edith Celley, who’s leading the efforts to preserve Haverhill’s oldest surviving public building. A consultant …