Restarting from checkpoint no longer reinitializes WL weights.

Restarting from checkpoint no longer reinitializes WL weights.

Fixes a problem where mdrun was reinitializing the initial Wang-Landau
delta for expanded ensemble simulations, because the flag turning it
off was stored the expanded ensemble data structure (not saved in cpt)
instead of the df_history structure (is saved in checkpoint).  In the
process, some moderate encasulaton of the df_history structure and
the expanded ensemble methods.

Fixes #1350

Change-Id: I13492a7a9773fcb417fcd0ee106d851d9838ce25