For people seriously ill with cancer, a positive outlook is essential. Yet so much of their time is spent in soul-destroying waiting rooms and strip-lit clinics. How much better for a patient to 'feel hugged by a building'. Simon Garfield reveals why Zaha Hadid's new Maggie's Centre - the architect's first commission on home soil - will be a fitting tribute to her late friend, Maggie Keswick Jencks.