When the Montreal singer-songwriter Patrick Watson was tapped to write music for “So Long, Marianne” — a new scripted series about the romance between Leonard Cohen and Marianne Ihlen — he had to find a way to channel Leonard’s music without imitating his sound. Patrick ended up writing the theme song for the series and his involvement didn’t end there. He joins Tom to talk about his work on “So Long, Marianne” and his relationship with the late Canadian icon.