Spiced with cumin and flavored with fresh herbs — Growing up in India, “Mattha” was a favorite summer beverage. Mattha is, essentially, spiced buttermilk. It is a mix of yogurt, water, fresh herbs — and sometimes, a smidgen of spice. Mattha in Mumbai — You walked in from the pitiless sun, insides broiling, every cell in your body raging with thirst. Head pounding…