Tom Estabrook gives a tour of his enormous new greenhouse that has all the latest technology in horticulture. POWNAL, Maine — Estabrook’s has two garden centers, one in Yarmouth and another in Kennebunk, but the family-owned business recently opened an enormous state-of-the-art greenhouse in Pownal. “[It’s] 55,000 square feet of new greenhouse,” Tom Estabrook boasted…