The sprawling new Katara Suite atop the Excelsior Hotel Gallia, a Luxury Collection Hotel, is an oasis from the bustling city of Milan. With more than 10,700 square feet of living space, it is the largest presidential suite in all of Italy. Guests enjoy two private outdoor terraces and sweeping views of the city’s historic Central Station. The biggest attraction, though, in the grand Studio Marco Piva–designed space is the arresting private spa with an enormous hot tub positioned beneath the building’s histori…