MNT : more re-factoring of guts of DM
MNT : more re-factoring of guts of DM

All logic for filling auxiliary data structures now lives in add_column
alone.

Logic for setting up empty data structures lives in both `__init__` and
`recreate_columns` mostly for documentation reasons (and to preserve the
ability to make this class slotted).

NSLS-II/replay,ericdill/replay,ericdill/replay,NSLS-II/replay