Eli Katzoff started out with a simple plan to grow some tomatoes at home. That was before the engineering plans, the kebab-stick model, the 16-foot long wood planks, the website and the ad-hoc community garden/charity. Katzoff’s idea, on display in the front yard of his childhood home near Route 9, looks like an oversized swing set, stretching 13 feet high. The only problem, according to Newton city officials, is the entire structure is also illegal.