Testgrad program needs to loop over archive structures.

[leeping]

In order for ForceBalance to do force matching calculations more efficiently, it would be ideal if the testgrad program could loop over a trajectory and calculate the forces on each frame.

Currently, testgrad needs to be called separately for each frame, which slows things down because the program gets loaded into memory, variables need to be initialized and so on.

Referring to #1, if we come up with a good way to include periodic boxes in the trajectory, it would be good for testgrad to take advantage of this as well.

[leeping]

TINKER 6.3 implements this, closing the issue. :)