New dmc

[lkwagner]

Addresses #42 by adding t-moves with accept/reject as suggested by Cyrus Umrigar. To do this, also:

• Reorganizes EnergyAccumulator to generate t-moves and avoid awkward calls to different energy functions.
• Fixes a bug in the random rotations of the ECP.
• Fixes the way that the trial energy is updated in DMC.
• Changes the reweight to enable smaller timestep errors.

Note that DMC runs done after this will NOT be compatible with previous DMC runs. If you try to restart an old run, it should detect it and raise an error.