writer function for atoms (TransientGridSearch)

Here's a simpler merge request that can go in before !13 (merged), just adding a write_atoms_to_file method for debugging purposes to the standard ComputeFstat class, wrapping lalpulsar.write_MultiFstatAtoms_to_fp().

It's another feature that I only really need for TransientGridSearch at the moment, but in principle other search classes could request the whatToCompute = lalpulsar.FSTATQ_ATOMS_PER_DET flag too, so I think it makes sense to have this at the ComputeFstat level.