Aldous Harding ’s music is warm and inviting, but it’s never quite clear who’s mailed you the invitation. The New Zealand singer-songwriter wanders around inside her psychedelic folk arrangements, singing in private riddles and changing the tone of her voice from one song (or even one verse) to the next, seemingly oblivious or indifferent to your presence. Despite the many distinct people she embodies, she never struggles to contain her multitudes. She’s like a veteran character actor: You forget, very quickly, to recognize her. On her career-best album, Train on the Island , Harding steps closer than ever to the camera lens without coming into focus.…