A group of cloistered nuns in rural Pennsylvania who trace their history back to 16th-century Spain are building a sprawling monastery meant to last 1,000 years — and they’re doing it almost entirely by hand. Expected to be completed in 2041 at a cost of $100 million, the 48-acre Carmel of Jesus, Mary and Joseph monastery in Fairfield, Pa., is entirely the vision of one woman — Mother Thérèse of Merciful Love, the 41-year-old daughter of a…