Update TODO.md
Update TODO.md

Added to ``Other single SN datasets'':
1998bu, need to reacquire spectrum on 1999-01-23 - 2001ApJ...549L.215C
lzkelley/sne,jparrent/sne,jparrent/sne,lzkelley/sne,lzkelley/sne,lzkelley/sne,jparrent/sne,jparrent/sne