KATHERINE HOURELD  THE Somali insurgent group al Shabaab collects almost as much tax as the government through a sophisticated system of levies on activities from importing goods to irrigating crops, a think tank report has found. The Mogadishu-based Hiraal Institute said al Shabaab's tax collectors were bringing in an estimated $15 million per month. Its report was based on more than 70 interviews with businessmen, government officials, al Shabaab defectors and active members of the insurgents' tax collection cell. It examined receipts and income reports from religious taxes and fees charged on activities such as crossing checkpoints, running businesses, irrigating…